Stressful events in life, such as the death of a child, divorce or being fired, can age the brain by at least four years, researchers suggest.
Researchers looked at memory and thinking performance of 1,300 people in their 50s to gauge their brain health.
The findings were presented at the Alzheimer's Association International Conference in London.
Although the research could not establish any direct link between stress and an increased risk of dementia, stressful experiences are known to have an impact on brain function, which could then lead to dementia in the longer term. The theory is that stress increases inflammation, which could increase the chances of developing dementia.
This study, from the University of Wisconsin School of Medicine, found that African Americans were more at risk of stress in life than other ethnic groups. This is because they scored poorer results in the memory tests than other groups and also tended to live in poorer neighborhoods.
Stressful experiences across all groups included financial insecurity, serious health problems and psychological trauma. Conditions such as anxiety and depression are also thought to contribute towards dementia risk. There is obvious link between stress and memory decline. The findings do indicate that more should be done to support people from disadvantaged communities who are more likely to experience stressful life events.
Dementia mainly affects people over the age of 65 and, while the likelihood of developing dementia rises sharply with age, many suffering from the condition are younger than that. Events and experiences throughout life can impact the brain decades later and researchers take a whole lifespan approach to understanding brain health in later life.
Tips for reducing the risk of dementia
Keep physically active for at least 30 minutes, five times a week.
Don't smoke, or give up if you do.
Eat a healthy balanced diet including oily fish, fruit, vegetables and low levels of carbs and sugar.
Keep your alcohol intake to a minimum.
Manage other health conditions such as high blood pressure, high cholesterol and type 2 diabetes.
Keep to a healthy weight to reduce your risk of type 2 diabetes, stroke and heart disease.
Give your brain a daily workout by doing puzzles, word searches or crosswords or learning something new.
Keeping socially engaged and have a good social network.