From the New York Times: "What, I’d like to know, will persuade the majority of Americans who remain sedentary to get off their duffs and give their bodies the workout they deserve? My hope is that every new testimonial to the value of exercise will win a few more converts until everyone is doing it. ... Physical inactivity is one of the strongest predictors of unsuccessful aging for older adults and is perhaps the root cause of many unnecessary and premature admissions to long-term care. ... [It has long been] well established that higher quantities of physical activity have beneficial effects on numerous age-related conditions such as osteoarthritis, falls and hip fracture, cardiovascular disease, respiratory diseases, cancer, diabetes mellitus, osteoporosis, low fitness and obesity, and decreased functional capacity. ... exercise [produces] a significantly reduced risk of cognitive impairment after two years for participants with moderate or high physical activity [when older] than 55. ... Sedentary skeptics are fond of saying that of course exercise is associated with good health as one ages; the people who exercise are healthy to begin with. But studies in which some participants are randomly assigned to a physical activity program and others to a placebo (like simply being advised to exercise) call their bluff. Even less exacting observational studies, like the Nurses' Health Study, take into account the well-being of participants at enrollment."

Link: http://www.nytimes.com/2010/03/02/health/02brod.html