It is certainly important to continue physical activity well into adulthood, given the wide-ranging benefits of exercise. Generally speaking, as we age it gets harder to maintain bone and muscle mass, and we tend to gain more fat. Most individuals reach their peak bone mass some time in their 20s. A steady, slow decline in bone mass usually occurs thereafter. Bones are constantly adapting to the forces on them, and weight-bearing exercise in moderation like walking, jogging or running stimulates the bones to stay strong. There's no convincing evidence that running is better for this than walking, so pick something you enjoy and can stick with. The bones need fuel to respond, so it's important to have enough calcium and vitamin D. Maintaining muscle mass and strength is also beneficial for overall health and injury prevention, so doing some kind of resistance training (e.g., weights, plyometrics) is important. In the already active 40- or 50-something (and above), it is important to pay attention to the normal declines in strength, muscle mass, bone mass, flexibility and ability to recover that come with aging, and adjust your workout routine accordingly. That often means special attention to a proper warm-up to activate the muscles and reduce injury risk, and to cool-down and stretching after activities. It may also mean breaking up the routine that worked for you in your younger years and mixing in some cross-training. Doing the same routine day in and day out can lead to patterns of overuse and injury. Mix it up with a variety of weight-bearing/impact (e.g., running) and nonweight-bearing (e.g., cycling, elliptical, rowing, [cross country] skiing) cardiovascular activities, as well as some strength training.