What are the causes of young-onset dementia?

The causes of young-onset, or early-onset dementia are similar to the diseases that usually cause dementia in older people. However, some causes, such as frontotemporal dementia (FTD), are more common in younger people. Dementia in younger people often has different symptoms, even when it’s caused by the same diseases as in older people.

There is more information about some common causes of dementia, and how they can affect younger people, below.

Alzheimer’s disease

Alzheimer's disease is the most common type of early-onset dementia and may affect around 1 in 3 younger people with dementia. However, up to 2 in 3 older people with dementia have Alzheimer’s disease. Alzheimer’s disease develops when proteins build up in the brain to form structures called ‘plaques’ and ‘tangles’. This causes brain cells to die and so affects functions controlled by the brain.

For most older people with Alzheimer’s disease, the first symptom they notice is likely to be memory loss. However, in younger people with Alzheimer’s disease, memory loss is less likely to be the first symptom.

Atypical Alzheimer’s disease

Younger people are much more likely to have an ‘atypical’ (unusual) form of Alzheimer’s disease. The symptoms depend on the specific form a person has:

  • In posterior cortical atrophy (PCA) the first symptoms are usually problems with understanding visual information, such as struggling to read or judge distances.
  • In logopenic aphasia the first symptoms are usually difficulties with language, such as problems finding the right word, or taking long pauses while they speak.
  • In behavioural/dysexecutive Alzheimer’s disease the first symptoms are usually difficulties with planning and decision making, and behaving in socially inappropriate ways.

Up to 1 in 3 younger people with Alzheimer’s disease have one of these forms, but only 1 in 20 older people with Alzheimer’s disease have an atypical form. For more information see Rarer types of dementia.

Familial Alzheimer's disease

Familial Alzheimer’s disease is a very rare form of Alzheimer’s disease. It is caused by genetic mutations (changes in genes) that run in families. Three genes have been found to have these rare mutations – PSEN1 (presenilin 1), PSEN2 (presenilin 2) and APP (amyloid precursor protein).

There is a 50% risk of a parent passing on the familial Alzheimer’s disease mutation to their children. People with the condition usually have a strong family history of the disease and will know a range of family members who were affected at a similar age, along with one of their parents.

Symptoms of familial Alzheimer’s disease usually start in someone’s 30s, 40s or 50s. The earlier the symptoms start, the more likely it is that the disease is genetic. Familial Alzheimer’s disease is very rare and probably accounts for fewer than 1 in 100 people with Alzheimer’s disease. For more information see Types of genetic testing.

Learning disabilities and dementia

People with Down’s syndrome and other learning disabilities are more likely to develop dementia when they are younger. Dementia in people with Down’s syndrome is usually caused by Alzheimer’s disease, but not always.

The reasons why people with Down’s syndrome are at increased risk of Alzheimer’s disease are not well understood. It is thought to be due to chromosome 21, which most people with Down’s syndrome have an extra copy of. This leads to an overdevelopment of a protein (amyloid precursor protein) which forms plaques on the brain. These plaques are a feature of Alzheimer’s disease. By the age of about 40, almost all people with Down’s syndrome have changes in the frontal lobe of their brain. However, not everyone will develop the symptoms of dementia.