The two economists call their paper "Mental Retirement," and their findings from the United States and 12 European countries suggest that the earlier people retire, the more quickly their memories decline.

"It's incredibly interesting and exciting," said Laura Carstensen, director of the Center on Longevity at Stanford University in California. "It suggests work actually provides an important component of the environment that keeps people functioning optimally."

Japan and South Korea have begun administering a survey on memory. China, India and several countries in Latin America are also planning surveys.