According to a WSJ article, November 16, 2006; Page D1, titled How to Keep Your Aging Brain Fit: Aerobics.

-“According to a new study, the brain’s long, slow decline may not be inevitable. For the first time, scientists have found something that not only halts the brain shrinkage that starts in a person’s 40s, especially in regions responsible for memory and higher cognition, but actually reverses it: aerobic exercise. As little as three hours a week of brisk walking — no Stairmaster required — apparently increases blood flow to the brain and triggers biochemical changes that increase production of new brain neurons.

-”Now he and colleagues have discovered what may be the basis for these improvements. As little as three hours a week of aerobic exercise increased the brain’s volume of gray matter (actual neurons) and white matter (connections between neurons), they report in the November issue of the Journal of Gerontology: Medical Sciences.

-”After only three months,” says Prof. Kramer, “the people who exercised had the brain volumes of people three years younger.” “This is the first time anyone has shown that exercise increases brain volume in the elderly,” says Dr. Kramer. “It suggests that aerobic exercise can stave off neural decline, and even roll back some normal age-related deterioration of brain structure.”