Once adults pass the physical prime of their teens and 20's, they lose an average of 10 ounces of lean body mass a year. This is mostly in the form of muscle tissue. Few people actually lose 10 ounces of weight a year. Instead, most gain about a pound a year, so the loss of lean tissue is masked.
Another way to look at this is the average person gains about 1 pound and 10 ounces of body fat per year. This phenomenon, called sarcopenia, is derived from Greek words for "vanishing flesh." This change in body composition contributes to impaired wound healing, loss of skin elasticity, and an inability to fight infection.