We lose lean muscle mass as we age.  Left unchecked this reduction in muscle mass leads to two bad outcomes.  The first bad outcome is reduction in metabolic rate.  Muscles burn more calories than fat.  If you lose muscle mass you lose the calorie burning effect of the muscles.  If your metabolic rate goes below your daily caloric intake you put on weight.  The second bad outcome is the loss of ability to move.  A good example is getting out of a chair.  If your legs, hips, back, and stomach muscles are not strong enough to move your body then you lose the ability to get up from a chair. 

Another aspect of aging is the reduction in bone density.  As we age, the density of the bones decrease - in men as well as women.  Resistance training stimulates bone growth, thus mitigating the loss of bone density through aging.  Clients who work with me maintain or increase their lean muscle mass and bone density.