Lifestyle changes to reduce a person's risk of dementia have to start in middle age, scientists say.

People who want to reduce their risk of dementia in later life have to start making lifestyle changes in middle age.

The critical period to optimise healthy brain ageing and preventing dementia is up to 20 years before symptoms appear, says Professor Sharon Naismith of the University of Sydney's Brain and Mind Centre.

She runs an intervention program at the centre that focuses on treating patients in the early stages of dementia.

The program targets conditions and behaviours that contribute to cognitive decline, including depression, sleep problems and inactivity.

Prof Naismith says while there is no cure for dementia, there are ways to reduce the risk.

"Studies show that 30 to 50 per cent of dementia risk is due to factors that can be addressed in midlife," she said.

"Physical activity, cognitive activity, depression and sleep disturbance are key targets in midlife, as well as those strategies focused on heart health."

Alzheimer's Australia says what is good for your heart is also good for your brain.

It says the risk factors associated with dementia can be managed through lifestyle changes.

About a quarter of a million Australians have dementia and this figure is expected to quadruple during the next four decades.

Economic modelling released this week shows more than one million will be diagnosed with dementia by 2056 at a cost of $1.033 trillion.

Prof Naismith says workplaces and society more broadly need to take responsibility to lessen the burden.

"The general public is increasingly worried about dementia as this is going to be an even bigger problem over the next three decades," she said.

"We really need society to get behind this issue now if we are going to make an impact."

LIFESTYLE MEASURES TO REDUCE DEMENTIA RISK

• Look after your heart - heart disease and midlife hypertension increase the risk of Alzheimer's disease and vascular dementia. People should work to lower their cholesterol and blood pressure.
• Increase physical activity - at least 30 minutes a day for adults is recommended.
• Eat healthy - the Mediterranean-style diet, high in omega-3, polyunsaturated and monounsaturated fats, is good. Reduce intake of saturated and trans fats.
• Avoid head injury - studies suggest moderate to severe head injuries increase the risk of developing Alzheimer's disease and other forms of dementia.
• Challenge your brain - learn new things such as dancing or a second language.
• Remain social - observational studies have shown an association between lower dementia risk and higher levels of social interaction.