Data from a large Chinese study of women 40-70 years old reveals that adolescent and adult exercise significantly reduces the risk of all-cause mortality. Women who didn't start exercising until adulthood saw a lower risk also, but not as low. Exercise is good at all ages, but there seems to be an additive benefit when started early in life.
PositiveTip: Girls who exercise put into their bank a lifetime benefit that lowers the risk of cardiovascular disease, cancer, and premature.