The World Health Organization (WHO) has launched its first ever guidelines on how people can help avoid getting dementia.
It looked at the evidence of what works and what doesn't for lowering risk.
Things to avoid include smoking and drinking too much alcohol.
While eating a healthy diet is beneficial, popping vitamin pills makes no difference to dementia risk, it advises.
There is no good evidence that brain training works but some studies suggest it may be worthwhile, says the WHO.
Dementia affects around 50 million people around the world and is becoming more common.
While there is no treatment that can cure it, there are things people can do to lower their risk of the disease or slow its onset.
Old age is the strongest risk factor, but it is not a natural or inevitable consequence of ageing.
Genetics also play a role, but many risk factors are modifiable.
Experts have previously suggested lifestyle changes could prevent around a third of dementia cases.
Cut your risk
The guidelines recommend:
1) Exercise - adults, including the elderly, should aim to get at least 150 minutes of moderate-intensity physical activity a week. This could include planned exercise, like cycling, or everyday activities such as housework
2) Stop smoking - it is bad for brain and body
3) Eat well - a healthy, Mediterranean-like diet containing plenty of fresh vegetables and fruit is beneficial
4) Don't bother with vitamin pills - there is no evidence that they help lower dementia risk
5) Avoid heavy alcohol use - drinking too much is risky. Some studies have suggested that light consumption might actually be protective against dementia but there is not enough good evidence to support this idea
6) Brain training - some studies suggest that activities to challenge the brain, such as crosswords and bespoke computer games, could be beneficial
7) Be social - although there is no proof that it will stop or slow dementia, staying connected with friends is linked with good health and wellbeing
8) Keep a healthy weight - this goes hand in hand with eating a good diet and getting enough exercise to stay fit and healthy
9) Beware high blood pressure - there is a strong link between dementia and high blood pressure
10) Get treated if you have diabetes - good control of blood sugar is important for lowering the risk of associated complications, including dementia
11) Beware high cholesterol - it is a risk factor, although it is not clear whether cholesterol-lowering medication (statins) will reverse the risk
Dr Carol Routledge, from Alzheimer's Research UK, welcomed the guidelines saying: "While we cannot change the genes we inherit, taking the steps outlined in this report can still help to stack the odds in our favour."
Fiona Carragher from the Alzheimer's Society said: "It's estimated a third of dementia cases could be prevented, and this report provides the best available prevention advice."
Prof Robert Howard, an expert at University College London, said: "We are probably decades away from treatments to slow or stop established dementia. Prevention would be so much better than a distant cure."