Can the brain heal and preserve itself—or even improve its functioning—as we get older? For some time, many scientists have tended to think of our brains as machines, most commonly as computers, destined to break down over time under the strain of age and use. In recent years, however, research in neuroscience has begun to show the inadequacy of this metaphor for describing the physiology of the brain. It turns out that our brains, like our bodies in general, are far more likely to waste away from underuse than to wear down from overuse.

As...