Just over half the participants showed a normal rate of age-related decline while 16% suffered a major reduction in their mental faculties. However, 30% of the study volunteers remained unchanged and in some cases even showed an improvement in performance over the years.
The researchers then examined what lifestyle factors stood out among those people who were able to remain quick-witted in old age.
Study leader Dr Alexandra Fiocco, from the University of California at San Francisco, said: "To this day, the majority of past research has focused on factors that put people at greater risk to lose their cognitive skills over time, but much less is known about what factors help people maintain their skills."
The research, published in the journal Neurology, revealed a unique profile that distinguished people who avoided mental decline with the passing years. Those who exercised moderately or vigorously at least once a week were 30% more likely to "stay sharp" than people who did not.
Individuals with a good education were nearly three times more likely to maintain their mental faculties than those with less education. Likewise, high levels of literacy were associated with a five-fold better chance of side-stepping age-related mental decline.
Non-smokers were nearly twice as likely to remain mentally fit than smokers. And people who were socially active - either by working or volunteering, or by living with someone - were 24% more likely to avoid mental deterioration in later life.
Not surprising perhaps but more confirmation of what we need to do to stay sane, fit and healthy.