After the age of 40, we may notice occasional lapses--a forgotten phone number, a friend's name, or a word that was right on the tip of our tongue. By 60, we may find ourselves wondering who called this morning, why we came into the kitchen, where we parked the car. In an aging nation, where one citizen in seven will be 65 when the next century arrives, these little difficulties raise a larger question: What precisely happens to our thinking as...