While it is not exactly news that those who enter retirement from their careers often find themselves bored and even depressed, it is only in recent years that research has focused on the apparent association of memory loss with leaving work. It is premature to suggest that one causes the other, yet the confluence seems real. Gina Kolata reports in Taking Early Retirement May Retire Memory, Too that cognitive skills do decline unless (like any skill) we keep them sharp through practice. " ... 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 ... You get better at one narrow task, but you don't get better at cognitive behavior in life."
Comparing memory function in countries with varying retirement ages, one study found that people who retire later retain more cognitive funtion (see graph below, click to enlarge). In this regard, Americans score well. Whether this is because they retire later in life, or whether the coincidence of memory retention and retirement are linked to a third, as yet unidentified factor, has yet to be explored.
When viewed as a function of information processing, the solution for those who do retire early would seem to be simply -- remain socially active, keep your personality skills sharp, and enter into complex activities which not only strengthen existing neural pathways, but establish new ones. Learn a new language, read books which challenge your thinking or assumptions (a great path is to join a book club in which the members read and discuss a given book each month -- the discussions serve as both social function and intellectual challenge), meet new people, or travel to places you've never been, preferably outside your own culture.
Our golden years should not be spent passively vegetating in a rocking chair. These can be the best times of our lives.