The words are on the tip of your tongue, but you just can't quite remember them. You've been there a hundred times, so why did you take the wrong turn? Outwardly, as people age they may be looking and feeling younger than their parents' generation--60 is the new 40, after all--but mental decline can begin as early as age 30, and it will impact everyone at some point. The increasing rate of dementia is sobering, and the personal, financial, and societal stakes are high.

The good news is, just like diet and exercise can keep aging bodies healthier, the proper mental regimen can...