Is it normal aging or dementia? MCI, dementia and Alzheimer’s explained

This is the second post in our series on Alzheimer’s, written for people living with the disease and the people who care for them. The first post set out what the disease is and how it shows up in daily life; before going further, we need a shared vocabulary. These terms are often used loosely, without matching what each one actually means. They describe different things: a process in the brain, a clinical syndrome, and a specific disease. This post tries to separate them, and to be honest about where the separation gets blurry.

Please note: this is an informational blog post written primarily for carers. Although we draw on academic sources and textbooks, it is not diagnostic or intervention guidance. For anything specific to you or someone you care for, please consult a specialist.

Key points

• Some change in thinking is a normal part of aging. Dementia is defined by a loss of independence in everyday tasks, not by any single memory lapse.

• Mild cognitive impairment (MCI) sits in between. Many people with MCI stay stable or return to normal; some go on to develop dementia.

• Neurodegeneration is a process, dementia is a syndrome (a pattern of symptoms), and Alzheimer’s is a disease, the most common cause of dementia.

• For most people, genes raise or lower the risk of Alzheimer’s but do not decide it. Huntington’s disease, also described here, is the clear exception.

• If you notice changes in someone close to you, what you have observed is valuable to a clinician. Only a specialist assessment can say what the changes mean.

Normal aging, mild cognitive impairment and dementia

Let’s start with something that is easy to lose sight of when you’re worried: some change is expected. Even people who never develop mild cognitive impairment or dementia can notice subtle shifts in how they think as they age, and these shifts can touch everyday life. This is part of why telling normal aging from disease is hard, even for clinicians (Harada et al., 2013). (“Normal” here describes a typical course, not an absence of change.) Is dementia a normal part of aging, then? No. Some change in thinking is expected, but dementia goes beyond what usual biological aging would produce (World Health Organization [WHO], 2025).

Clinical frameworks rarely draw the line by what is forgotten. They ask what the forgetting does to a person’s independence. The DSM-5-TR distinguishes a mild neurocognitive disorder, a modest decline in one or more cognitive domains, from a major one, the category that corresponds to what most people call dementia. In the mild form, complex everyday tasks such as paying bills or managing medication are still possible, though they may take more effort, workarounds or accommodation. In the major form, the person needs help with at least some of these tasks (American Psychiatric Association, 2022). In other words, dementia is defined by a threshold in function, not by a particular memory lapse.

Mild cognitive impairment (MCI) sits between the two, and it is common. Estimates from a 2018 guideline range from about 7% of people aged 60–64 to about 25% of those aged 80–84 (Petersen et al., 2018). Does mild cognitive impairment always lead to dementia? No, and what happens next is the uncertain part. A meta-analysis of 89 studies with an average follow-up of 5.2 years found that 41.5% of people with MCI in clinical samples went on to develop dementia, compared with 27.0% in population-based samples. About half stayed stable, and some returned to normal cognition: 8.7% in clinical samples and 28.2% in population-based ones (Salemme et al., 2025). Who is studied changes the answer, because people who reach a memory clinic differ from people in the general community. MCI is therefore better read as a position than a destination. When it is attributed to Alzheimer’s-related changes, it is called “MCI due to Alzheimer’s disease” (Albert et al., 2011), but the label alone cannot tell us which path a person will follow.

Because the line is drawn at function, early signs are easy to explain away. Researchers note that cognitive and functional changes are often perceived as part of normal aging or as not serious enough to need attention, which delays help-seeking and diagnosis (Kusoro et al., 2025). A recent meta-analysis pooling ten studies estimated an average of 3.5 years between the first noticed symptoms and a diagnosis of dementia, and 4.1 years for young-onset dementia (Kusoro et al., 2025). These figures rest on retrospective reports from patients and carers, and definitions of “symptom onset” vary between studies. Most of the studies came from Europe, the US and Australia, and the authors caution that their findings are unlikely to apply to low- and middle-income countries. So we cannot assume this picture holds everywhere.

This raises the next question. If dementia describes what a person can no longer do, what is happening in the brain to cause it, and what do we call that?

How neurodegeneration, dementia and Alzheimer’s disease differ

Neurodegeneration, dementia and Alzheimer’s are related, but in everyday use they are often applied to things they don’t describe. Each answers a different question, and using one where another belongs causes much of the confusion around memory loss.

Neurodegeneration is a process: the progressive loss of neurons and their function. It is better understood as an umbrella than as a single condition. It covers a large group of neurological disorders that differ in how they look clinically and pathologically, each affecting particular subsets of neurons in particular functional systems (Przedborski et al., 2003).

Dementia is a syndrome, meaning a pattern of difficulties rather than a cause. The WHO describes it as a decline in cognitive function beyond what usual biological aging would produce, affecting things like memory, thinking, orientation, language and judgment. It is not one illness but the result of a variety of diseases and injuries that affect the brain (WHO, 2025). A cough and a fever describe what you see, while pneumonia names what is behind them. Dementia is closer to the first, and Alzheimer’s disease is closer to the second.

Alzheimer’s disease is a specific disease and the most common cause of dementia. The WHO estimates that it contributes to 60–70% of cases among the 57 million people who were living with dementia worldwide in 2021 (WHO, 2025).

Some neurodegenerative diseases show up mainly as problems of movement. Parkinson’s disease is the best-known example. The DSM-5-TR also lists Parkinson’s and Huntington’s disease among the conditions that can lead to a major neurocognitive disorder, so some people go on to develop dementia (American Psychiatric Association, 2022).

Dementia can also follow a stroke, or develop in the context of certain infections such as HIV, harmful use of alcohol, repeated head injuries or nutritional deficiencies (WHO, 2025). A meta-analysis of studies published up to 2002 found that vascular causes accounted for about a fifth of dementia cases, second only to Alzheimer’s. Can dementia be reversed? Rarely. The same meta-analysis found that potentially reversible causes were identified in roughly 9% of cases, but only about 0.6% actually reversed (Clarfield, 2003). Those numbers are a reminder that causes differ, and that finding out which one is at work is exactly what a specialist assessment is for.

Even among the forms of dementia, the WHO notes that the boundaries are indistinct and that mixed forms often co-exist (WHO, 2025).

Alzheimer’s disease therefore sits where all three meet. It is a neurodegenerative disease that most often reaches us as the syndrome of dementia, and it can also be present at the earlier stage of mild cognitive impairment (Albert et al., 2011).

So is Alzheimer’s a type of dementia? Strictly, Alzheimer’s is a disease, and dementia is the pattern of symptoms it most often causes. Calling Alzheimer’s “a type of dementia” is shorthand for that (WHO, 2025).

What neurodegeneration is

The word is plainer than it looks: neuro- for nerve cells, degeneration for a gradual loss of structure or function. Read strictly, it means any disease that primarily affects neurons (Przedborski et al., 2003). In practice it names a large and varied group of disorders, each damaging particular neurons in particular systems.

Neurodegeneration and dementia are related but not the same. Neurodegeneration is one of the processes that can cause dementia, and dementia can also have causes that are not neurodegenerative, such as a stroke (WHO, 2025).

Researchers have recently proposed describing these diseases by what they share. One influential framework lists eight hallmarks: pathological protein aggregation (proteins clumping together), synaptic and network dysfunction, faulty protein handling, cytoskeletal abnormalities, altered energy metabolism, DNA and RNA defects, inflammation, and neuronal death. It suggests that individual diseases could be categorized by which hallmark dominates, and that a single person may show several at once (Wilson et al., 2023).

Two features set neurodegeneration apart from many other conditions. First, it tends to progress irreversibly, and aging is the primary risk factor for most of these diseases. Few or no treatments exist that stop or reverse them (Hou et al., 2019). So the practical goal is usually to slow the course.

Second, it is quiet for a long time. In Alzheimer’s disease, the underlying process is thought to begin years, if not decades, before dementia can be diagnosed (Sperling et al., 2011). By the time changes are noticed, a lot has already happened.

What starts the process is still not well understood. Despite substantial effort, a 2020 review still described a paucity of information about the causes of neurodegeneration (Peña-Bautista et al., 2020). What we do have is a good picture of risk. A risk factor is something linked to a higher chance of developing a condition across a group of people; it is not proof of a cause in any one person. Age is the strongest risk factor. For dementia specifically, the 2024 Lancet Commission identified 14 modifiable (potentially changeable) risk factors across the life course and estimated that addressing them could prevent or delay about 45% of cases (Livingston et al., 2024). That figure describes populations, not what will happen to any one person, and it assumes the factors are causal and could be fully removed. We look at the risk factors in detail later in this series.

Whether a factor is a cause or a consequence is itself debated, and chronic stress is an example. A narrative review linked chronic stress with faster progression and worse symptoms in Alzheimer’s, Parkinson’s and Huntington’s disease. It also looked at the opposite direction, how receiving a diagnosis affects stress, and concluded that more research is needed (Peña-Bautista et al., 2020). Causes and effects may loop back on each other, which is one reason carers’ own wellbeing is part of this series.

Alzheimer’s, Parkinson’s and Huntington’s disease compared

Many diseases sit under the neurodegeneration umbrella. Three of them account for most of what people hear about, and they show how different the family can look. The table summarizes them, and the paragraphs below add detail.

Alzheimer’s disease is the one this series is about. It is the most common cause of dementia and has its own section below.

Parkinson’s disease is the second most common neurodegenerative disease. It is best known for slowness of movement, muscle rigidity and tremor at rest, and its cardinal motor symptoms stem from the loss of dopamine-producing neurons in a midbrain region called the substantia nigra (Minakaki et al., 2020). Why those particular neurons are so vulnerable is still unexplained (Minakaki et al., 2020), and the cause of the disease remains unknown (Kalia & Lang, 2015). It is no longer seen as purely a movement disorder. Non-motor features such as autonomic problems (affecting automatic body functions such as blood pressure and digestion) and cognitive problems are now recognized as part of it (Kalia & Lang, 2015). Reported prevalence in Asian countries is slightly lower than in Western countries. Differences in study methods, diagnostic criteria and case-finding make comparisons difficult, so it is hard to say how much of the gap is real (Muangpaisan et al., 2009).

Huntington’s disease differs in a way that matters for questions about family history. It is a dominantly inherited condition caused by an expanded CAG repeat, a stretch of DNA in which three “letters” are repeated too many times, in the huntingtin gene on chromosome 4. Whether carrying the gene change leads to the disease, called penetrance, depends on the length of the repeat. With 40 or more repeats the disease is fully penetrant, meaning it develops in everyone who carries it and lives long enough; with 36 to 39 repeats penetrance is reduced, so some carriers develop the disease and others do not (McColgan & Tabrizi, 2018). A longer repeat means earlier onset, and the mean age of onset is in mid-adulthood, roughly 30 to 50 (Roos, 2010). The disease brings together movement, thinking and psychiatric changes. Damage begins in the striatum, a structure deep in the brain involved in movement and behavior, and spreads to the cortex. There is currently no disease-modifying treatment, that is, none that slows or stops the disease itself (McColgan & Tabrizi, 2018). Huntington’s is the clearest case where the question about family history has a direct answer; most of the other diseases are not like that.

Beyond these three, the list goes on. Reviews of the most commonly encountered neurodegenerative diseases also include frontotemporal dementia, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies and multiple system atrophy. Each has its own epidemiology, symptoms, pathology and management, which is why diagnosing them accurately is difficult (Erkkinen et al., 2018). Amyotrophic lateral sclerosis (ALS) is another. It is a progressive paralytic disorder in which motor neurons in the brain and spinal cord degenerate (Brown & Al-Chalabi, 2017). Several of these can also cause dementia, and the next post in this series covers the main types.

Alzheimer’s disease: an overview

Alzheimer’s disease is a progressive neurodegenerative disease and the most common cause of dementia. The WHO estimates that it contributes to 60–70% of dementia cases (WHO, 2025), which is why dementia statistics so often get quoted as if they were Alzheimer’s statistics. Pathologically, that is, by what is seen in brain tissue, it is defined by two protein changes in the brain, amyloid plaques and neurofibrillary tangles (which contain a protein called tau), together with the loss of neurons (DeTure & Dickson, 2019). Alois Alzheimer first reported the pattern in 1906 to a meeting of German psychiatrists, describing a patient whose illness he had followed to autopsy (Hippius & Neundörfer, 2003).

Symptoms beyond memory

The most common presentation begins with trouble learning and recalling new information, but it is not the only one. Diagnostic criteria also recognize presentations in which language, spatial perception (such as recognizing faces or objects), or reasoning and judgment are affected first (McKhann et al., 2011). Neuropsychiatric symptoms, such as changes in mood, behavior or perception, and a decline in everyday activities are part of the picture too (Liu et al., 2019). For anyone caring for a person with Alzheimer’s, this is useful to know, because the first signs may not look like what we expect.

Causes and hypotheses

The amyloid hypothesis, which places the buildup of amyloid first in the chain of events, is the most heavily tested explanation in clinical trials. Other hypotheses point to tau, cholinergic dysfunction (problems in brain cells that use the chemical messenger acetylcholine), mitochondrial problems, vascular changes and inflammation, and the ultimate cause is still described as obscure (Liu et al., 2019). We come back to the mechanisms, and the scientific debate around them, later in this series.

Alzheimer’s also rarely comes alone. Many people diagnosed with probable Alzheimer’s disease have mixed pathology, meaning other kinds of brain damage, such as damage to blood vessels, alongside plaques and tangles (Schneider et al., 2009).

Genetics and family history

Many people reading this will have a family history in mind, and here the answer is more nuanced than for Huntington’s. Genes matter. The strongest common genetic risk factor is a version of the APOE gene called APOE ε4 (Scheltens et al., 2021). Twin studies put the heritability of Alzheimer’s at 60–80% (Scheltens et al., 2021). Only a very small share of cases, often estimated at around 1%, are caused by single-gene mutations that pass down families in a predictable way (Bermejo-Pareja & del Ser, 2024; DeTure & Dickson, 2019).

A caution on reading this number. Heritability is a population statistic. It estimates how much of the variation in risk across a group of people is linked to genetic differences between them, in the populations that were studied. It does not mean that 60–80% of any one person’s risk comes from their genes, and it can differ between populations and environments (Visscher et al., 2008).

If a parent had Alzheimer’s, does that mean you will get it? Not necessarily. For the common late-onset form, genes add to risk rather than deciding it, and only the rare single-gene forms pass down families in a predictable way (Bermejo-Pareja & del Ser, 2024; Scheltens et al., 2021). Nothing in this post can tell any individual what their own risk is. That is a question for a specialist. We return to genes and risk later in this series.

That one disease can look so different, in its symptoms, its brains and its causes, is part of why naming and classifying it is harder than it seems.

Why diagnostic labels are still changing

The best-known neurodegenerative diseases carry the names of the physicians who first described them: James Parkinson in 1817 (Parkinson, 1817), George Huntington in 1872 (Huntington, 1872) and Alois Alzheimer in 1906, long before anyone could see what was happening inside the cells. These labels began as names for patterns that someone observed, not for the mechanism that causes them, and the boundaries have been redrawn several times since (Bermejo-Pareja & del Ser, 2024; Goedert & Spillantini, 2006). We trace that history later in this series.

Under the microscope, the boundaries are blurred too. Alzheimer’s is described as a mixed protein disorder (amyloid and tau) that is frequently accompanied by cerebrovascular disease and Lewy body disease (DeTure & Dickson, 2019), and many neurodegenerative diseases share clinical features, which is why accurate diagnosis is difficult (Erkkinen et al., 2018). Brain changes and symptoms also don’t always match: some people have Alzheimer’s changes in the brain and no symptoms in life (Bennett et al., 2006).

In 2024, this question came to a head. The Alzheimer’s Association’s revised criteria define Alzheimer’s as a biological process that begins while people are still without symptoms, so that an abnormal result on a “Core 1” biomarker, a measurable biological sign such as amyloid, or a form of tau linked to it, detected in spinal fluid, blood or a brain scan, is enough to establish the diagnosis (Jack et al., 2024). The International Working Group disagreed. It points out that most cognitively normal people with a positive biomarker will not become symptomatic in the near term, and argues they are better described as at risk (Dubois et al., 2024). The two groups largely agree on what the biomarkers measure; they disagree on what a positive result means for a person without symptoms.

None of this makes Alzheimer’s less real, and it doesn’t make the suffering it causes any less serious. It does mean that the words we use are tools still being refined. For someone worried about a parent, a partner or themselves, this raises a practical question: what should we do with so much uncertainty?

When to see a doctor about memory changes

You may have come to this post with a worry about someone you love, or about yourself. The sections above explain why that worry can be hard to resolve. The words are used loosely, the boundaries are still being drawn, and the same brain changes can look very different from person to person. A few practical points follow from this. None of them replaces an assessment by a specialist.

Noticing change is not the same as diagnosing it. Forgetting alone does not mean dementia. Clinicians look at whether there has been a decline from the person’s own previous level, how far it interferes with independence in everyday tasks, and whether something else, such as delirium or another mental health condition, better explains it (American Psychiatric Association, 2022). So a change from the person’s own usual level that is starting to affect everyday tasks, or any change that worries you or them, is a reasonable reason to talk to a doctor. Normal aging brings subtle changes of its own (Harada et al., 2013), and mild cognitive impairment does not always progress (Salemme et al., 2025).

What you have observed matters. Assessment typically combines a history from the person and from a knowledgeable informant with objective cognitive testing (McKhann et al., 2011). For a carer, being that informant is a real contribution. What changed, roughly when, and how it affects daily life are exactly the details a clinician needs.

Waiting has a cost, but readiness matters too. As we saw at the start, the gap between first symptoms and diagnosis is often years. The authors of that review prefer the idea of a timely diagnosis to simply an early one, because people and families differ in how ready they are to seek it. They also note that evidence on the direct benefits and harms of earlier diagnosis is still limited. Both things can be true: the delay is long, and the decision about when and how to seek an assessment is a personal one.

Ask what a label rests on. Because definitions differ, it is reasonable to ask a specialist whether a diagnosis is based on symptoms, on tests, or on both, and what it can and cannot predict. Assessment also matters because some contributing conditions are treatable, even though few cases fully reverse (Clarfield, 2003).

Carers need care too. Families and friends provide most of the care for people with dementia, spending on average about five hours a day, and women provide roughly 70% of carer hours (WHO, 2025). That is where this series goes next.

In the next post, we map the types of dementia, from vascular to Lewy body to frontotemporal, with a comparison table and a report on what we know about global prevalence. Later posts look at risk factors, detection and the scientific debates, and then the series moves from the science toward the care itself: living with the diagnosis, and caring for the people who care.

This post has tried to draw a map, and parts of it are still being drawn.