I attended a lecture recently by some folks from the Human Performance Lab at Ball State, and learned some interesting things about frailty and fitness. In terms of physical fitness as measured by VO2max and other parameters, there is a fairly clear "frailty threshold." While it's possible to boost your fitness level numbers at almost any age (from 10 to 30 percent), if you dip below the frailty threshold, you're not coming back. Obviously you can't outrun time forever, but there are a lot of folks in their sixties and seventies who flirting dangerously close to that threshold simply because of a sedentary lifestyle. One unlucky illness or injury, and over they go.
Exercise is the cheapest and most beneficial prescription around.