Keeping mentally active by reading books or writing letters helps protect the brain in old age, a study suggests.
A lifetime of mental challenges leads to slower cognitive decline after factoring out dementia's impact on the brain, US researchers say.
The study, published in Neurology, adds weight to the idea that dementia onset can be delayed by lifestyle factors.
An Alzheimer's charity said the best way to lower dementia risk was to eat a balanced diet, exercise and stay slim.
In a US study, 294 people over the age of 55 were given tests that measured memory and thinking, every year for about six years until their deaths.
They also answered a questionnaire about whether they read books, wrote letters and took part in other activities linked to mental stimulation during childhood, adolescence, middle age, and in later life.
After death, their brains were examined for evidence of the physical signs of dementia, such as brain lesions and plaques.
The study found that after factoring out the impact of those signs, those who had a record of keeping the brain busy had a rate of cognitive decline estimated at 15% slower than those who did not.
Dr Robert Wilson, of Rush University Medical Center in Chicago, who led the study, said the research suggested exercising the brain across a lifetime was important for brain health in old age.
He told BBC News: "The brain that we have in old age depends in part on what we habitually ask it to do in life.
"What you do during your lifetime has a great impact on the likelihood these age-related diseases are going to be expressed."
Dementia exacts a heavy toll on society, with more than 820,000 people in the UK alone currently living with the condition.
Commenting on the study, Dr Simon Ridley, head of research at Alzheimer's Research UK, said there was increasing evidence mental activity may help protect against cognitive decline. But the underlying reasons for this remained unclear.
"By examining donated brain tissue, this study has shed more light on this complex question, and the results lend weight to the theory that mental activity may provide a level of 'cognitive reserve', helping the brain resist some of the damage from diseases such as Alzheimer's," he said.
Dr James Pickett, head of research at the Alzheimer's Society, added: "More research and bigger studies are needed, but in the meantime reading more and doing crosswords can be enjoyable and certainly won't do you any harm.
"The best way to reduce your risk of developing dementia is to eat a balanced diet, exercise regularly and maintain a healthy weight."