Sports are useful at any age for muscle building. Many studies prove it, and it has recently been shown that the same amount of effort can be expended whether we are 25 or 75. Certain types of exercise build more muscle than others, however. For a long time, we believed that light endurance training (jogging, biking, swimming) was the best way to maintain our muscle mass as we aged. In fact, studies show that alternating between endurance and high intensity exercises works best.
Resistance training (first with your bodyweight, and later on with free weights) is a good way to begin training. Interval training, in which short periods of intense effort are alternated with longer endurance periods (no matter the sport), is also very effective as it promotes growth hormone production. The one to two percent of muscle mass which we lose every year on average after 40 can therefore be recovered within a few weeks of training, by training three to four times a week.