Taking Early Retirement May Retire Memory, Too

This is "probably" a fact (I have seen it happen) - here. It's not use it or lose it - it's that neural plasticity at works (may be that's a scientific euphemism):
“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.

And research has failed to support the premise that mastering things like memory exercises, crossword puzzles and games like Sudoku carry over into real life, improving overall functioning.

“If you do crossword puzzles, you get better at crossword puzzles,” said Lisa Berkman, director of the Center for Population and Development Studies at Harvard. “If you do Sudoku, you get better at Sudoku. You get better at one narrow task. But you don’t get better at cognitive behavior in life.”

The study was possible, explains one of its authors, Robert Willis, a professor of economics at the University of Michigan, because the National Institute on Aging began a large study in the United States nearly 20 years ago. Called the Health and Retirement Study, it surveys more than 22,000 Americans over age 50 every two years, and administers memory tests.

That led European countries to start their own surveys, using similar questions so the data would be comparable among countries. Now, Dr. Willis said, Japan and South Korea have begun administering the survey to their populations. China is planning to start doing a survey next year. And India and several countries in Latin America are starting preliminary work on their own surveys.