From Scientific American:
Previous studies have shown that engaging in brain-building activities is associated with a delay in late-life cognitive decline. But why? Does flexing the old gray matter somehow buffer against age-related intellectual impairment? Or is cognitive loss simply a consequence of the aging brain’s physical decline? To find out, researchers questioned nearly 300 elderly individuals about their lifelong participation in intellectual pursuits—like reading books, writing letters and looking things up in the library. Then, every year, for an average of six years until they died, the subjects took tests to measure their memory and thinking. What the researchers found is that folks who worked their mental muscles, both early and late in life, remained more intellectually limber than those who didn’t—even when a post-mortem look at their brains revealed the telltale signs of physical decline.
More here.