Age is the strongest known risk factor for dementia.

Whilst it is possible to develop the condition earlier - at least 1 in 20 people with dementia developed it at age under 65 - the chances of developing dementia rise significantly as we get older.

Above the age of 65, a person's risk of developing Alzheimer's disease or vascular dementia doubles roughly every 5 years.

It is estimated that dementia affects one in 14 people aged over 65, and affects one in six people aged over 80.

This may be due to factors associated with ageing, such as:

• higher blood pressure
• increased risk of cardiovascular diseases (e.g. heart disease and stroke)
• changes to nerve cells, DNA and cell structure
• loss of sex hormones after mid-life changes
• the weakening of the body's natural repair systems
• changes in the immune system.

Scientists have known for some time that the genes we inherit from our parents can affect whether or not we will develop certain diseases. The role of genes in the development of dementia is not yet fully understood, but researchers have made important advances in recent years.

More than 20 genes have been found that do not directly cause dementia but affect a person's risk of developing it. For example, inheriting certain versions (variants) of the gene apolipoprotein E (APOE) increases a person's risk of developing Alzheimer's disease.

Having a close relative (parent or sibling) with Alzheimer's disease increases your own chances of developing the disease very slightly compared to someone with no family history. However, it does not mean that dementia is inevitable for you.

It is also possible to inherit genes that directly cause dementia, although these are much rarer than the risk genes like APOE.

In affected families there is a very clear pattern of inheritance of dementia from one generation to the next. This pattern is seen in families with familial Alzheimer's disease (a very rare form of Alzheimer's which appears usually well before the age of 60) and genetic frontotemporal dementia.

If a person has the faulty gene then each of their children has a 50 per cent chance of inheriting it and so developing the dementia. 

Women are more likely to develop Alzheimer's disease than men. This is the case even if we allow for the fact that women on average live longer. The reasons for this are still unclear.

It has been suggested that Alzheimer's disease in women is linked to a lack of the hormone oestrogen after the menopause. However, controlled trials of hormone replacement therapy (HRT, which replaces female hormones) have not been shown to reduce the risk of developing Alzheimer's.

The age at which HRT is started, however, may affect the outcome. HRT (prescribed mainly to help with symptoms of the menopause) is not recommended as a way for women to help reduce their risk of dementia.

For most dementias other than Alzheimer's disease, men and women have much the same risk. For vascular dementia, men are actually at slightly higher risk than women. This is because men are more prone to stroke and heart disease, which can cause vascular and mixed dementia.

There is some evidence that people from certain ethnic communities are at higher risk of dementia than others. For example, South Asian people (from countries such as India and Pakistan) seem to develop dementia - particularly vascular dementia - more often than white Europeans.

South Asians are well known to be at a higher risk of stroke, heart disease and diabetes, and this is thought to explain the higher dementia risk.

Similarly, people of African or African-Caribbean origin seem to develop dementia more often. They are known to be more prone to diabetes and stroke.

All of these effects are probably down to a mix of differences in diet, smoking, exercise and genes.