Chris Crowley has the right idea. He doesn't delve, however, into scientific details.

You learned about Crowley in this column last October. He's the guy who went from a lawyer at a prestigious New York City firm to a ski bum to the co-author of two books on aging well, the best-known being Younger Next Year: Live Strong, Fit, and Sexy Until You're 80 and Beyond. As a result, Crowley now spends much of his time speaking to groups of graybeards on how to become "functionally younger."What he preaches primarily is movement and exercise preferably six sessions of 45 minutes of aerobic exercise and two to three sessions of weightlifting every week in order to stimulate the body's cells. If you don't keep the body moving as you age, he says, cells wither and die at a far faster rate.Crowley is living proof of which he speaks. He's 74, but he could easily pass for 65 even younger if he wears a tie to conceal the loose skin around his neck.He just doesn't know or want to burden his listeners with the details of why his advice works.It's all in your telomeres.The active end of your chromosomes are covered in layers of this protein-based material. As you age, the telomeres waste away, expose the active ends of the chromosomes, and this exposure makes cell division impossible.Then, when the cell dies, they can no longer replace itself.But guess what exercise does? It reduces the rate at which the telomeres waste away, keeping, in essence, a 40-, 50-, or 60-year-old exercising body biologically younger than one that doesn't.This column touched upon this concept before when German research found a 40-percent difference in telomere length in 50-year-old men who ran 50 miles a week as opposed to inactive men in their 50s. Furthermore, the 50-year-old runners' telomeres were almost the same length as far younger runners who compete for Germany's national team.In February, RoadBikeRider.com restated this research in its weekly newsletter while mentioning two other studies that strengthen the exercise-delays-aging theory: a study of twins that showed the one who exercised regularly had longer telomeres than the one who didn't, and another that estimated that regular exercise added 12 years of life when compared to not exercising.This reporting may have been triggered by the January issue of Archives of Internal Medicine that ran four count 'em four! studies that led to what you now see, I hope, as an inevitable conclusion: exercise is crucial for thriving as you age.And some of these studies, unlike the aforementioned study of runners, did not call for extreme amounts of exercise.One, based on data from the Nurses' Health study, found a correlation between how physically active women were in middle age and how healthy they were at age 70. Even moderate walking improved health and lowered the incidence of chronic diseases, heart trouble, and mental impairment.Another found moderate-to-intense exercise reduced the risk of mental impairment in both men and women over 55.This evidence and more like it led James O. Hill, professor of pediatrics and director of the Center for Human Nutrition at the University of Colorado at Denver, to say, "Exercise is better than any drug or anything else we have for aging.There is no downside. If [exercise] were a drug, it would be the safest, most effective drug in the universe."But unlike a drug, there seems to be little fear of an overdose.Proof of this comes in the 20 years of research Paul Williams, a scientist at Lawrence Berkeley National Laboratory has done on runners. While working with runners, he has found that the more miles they run (up to 50 miles a week), the less likely they are to develop heart disease or have strokes.Williams, however, feels that the increased benefits would continue over the 50-mile-per-week limit. It's just that he's had a difficult time finding enough runners logging that sort of mileage to interpolate their data.Which brings us to a crucial point: Just how much exercise is enough?Williams thinks the government's recommendation of 150 minutes per week is too low. Crowley's suggestion of six 45-minute aerobic sessions and two to three weightlifting workouts every week is probably what's needed if your goal for exercise is not to lose weight or improve mood but to keep your body functioning years younger than its given age.