Even if you’re 50 and have never taken part in a physical activity, a brisk half-hour walk three times a week can “basically reverse your physiological age by about ten years,” says Gareth Jones, director of the Canadian Centre for Activity and Aging in London, Ont.

His source? A three-year study of 220 retirement-age men in which half didn’t exercise and the other half walked briskly for 30 minutes three times a week. After a year, the exercise group showed a 12 percent increase in aerobic power and a ten percent increase in strength and hip flexibility—equivalent to what they would have lost over a decade had they not exercised at all.