Is is a simple fact. As we age we lose muscle and strength. There's even a medical term for this, sarcopenia. It's derived from the Greek words "sarcos" (flesh) and "penia" (lack of).
"Estimates of how much muslcle is lost with age vary from 8 percent to about 50 percent of our muscles. Men seem to lose muscle faster than women. Strength is lost more rapidly than muscle. Why is this important? When muscles get smaller, they get much weaker. Lost of strength is consistent with loss of mobility and independence, and the need for institutional care. Protein intake - amount, type and timing through the day - seem to play a role. The type of protein you eat also seems to play a role in preventing muscle loss. Dietary protein is made up of many types of amino acids. The amino acid leucine has been shown to preserve body muscle. Leucine is found in higher amounts in animal foods: beef, lamb, pork, poultry, fish, eggs, milk, and products made with milk. It's also found in soybeans and, to a lesser extent, other beans, nuts, and seeds." -- Jennifer K. Nelson, R.D., L.D. (Beef it Up to Prevent Muscle Loss)