If you like to play chess, bridge or a musical instrument, do not let old age stop you. A new study, conducted over two decades, concludes that regular exercise - the kind that challenges the mind - appears to ward off dementia.

The research adds to a growing body of evidence that exercising the mind reduces the chances of Alzheimer's and other forms of dementia related to old age.

The latest study, conducted by researchers at the Albert Einstein College of Medicine in New York, found that mentally active seniors reduced their risk of dementia by as much as 75 percent, compared to those who do not stimulate their minds.

Experts agree that physical exercise is necessary for a healthy lifestyle, regardless of age. But the researchers also looked at whether physical activity contributes to mental agility.

Albert Einstein neurologist Joe Verghese, who led the study, said most of the types of physical exercise they looked at did not appear to significantly reduce the risk of dementia in older adults, with one exception.

'It turns out that one of the 11 activities, the physical activities we studied, that was associated with a significant risk reduction when looked at one-by-one, and that was dancing. It may be that dancing is not purely physical, in the sense that it does involve a lot of mental effort,' he said.

The benefit of mental exercise among seniors was seen in all of the elderly participants, regardless of their intelligence.

During the course of the 21-year study, researchers followed more than 450 people over the age of 75, beginning in 1980. None of the participants suffered from dementia at the start. Investigators tracked how many of the participants developed Alzheimers disease and other forms of dementia, and looked at what activities they had engaged in.

The seniors who remained mentally active showed the least decline in mental acuity.

James Coyle is a professor of psychiatry and neuroscience at Harvard University Medical School in Boston, who wrote an independent review of the study. Dr. Coyle said there is growing evidence that mental exercise may do more than forestall dementia, it may actually repair the brain, as well.

'And that as we use the brain to do particular things, it rewires itself. Perhaps the most interesting thing is the evidence that has accrued over the last couple years that there are stem cells in the brain, and that, in the adult brain, new neurons can be generated,' Dr. Coyle said.

The results of the Albert Einstein study were published in the New England Journal of Medicine.