Gina Kolata in The New York Times:
The two economists call their paper “Mental Retirement,” and their argument has intrigued behavioral researchers. Data from the United States, England and 11 other European countries suggest that the earlier people retire, the more quickly their memories decline. The implication, the economists and others say, is that there really seems to be something to the “use it or lose it” notion — if people want to preserve their memories and reasoning abilities, they may have to keep active. “It’s incredibly interesting and exciting,” said Laura L. Carstensen, director of the Center on Longevity at Stanford University. “It suggests that work actually provides an important component of the environment that keeps people functioning optimally.” While not everyone is convinced by the new analysis, published recently in The Journal of Economic Perspectives, a number of leading researchers say the study is, at least, a tantalizing bit of evidence for a hypothesis that is widely believed but surprisingly difficult to demonstrate.
Researchers repeatedly find that retired people as a group tend to do less well on cognitive tests than people who are still working. But, they note, that could be because people whose memories and thinking skills are declining may be more likely to retire than people whose cognitive skills remain sharp.