Staying mentally active by reading books or writing letters helps protect the brain in old age, says a new American study.

The study, published in Neurology, says performing mental challenges leads to slower cognitive decline. It adds weight to the idea that dementia onset can be delayed by lifestyle factors, BBC reported.

In the study by the Rush University Medical Center in Chicago, 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 percent slower than those who did not.

Robert Wilson, of the Rush University Medical Center, who led the study, said the research suggested exercising the brain across a lifetime was important for brain health in old age.

"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," Wilson told BBC.

Commenting on the study, Simon Ridley, head of research at Alzheimer`s Research UK, said there was increasing evidence that 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 told BBC.