Loss of muscle begins in the 30s as Testosterone and Growth Hormone begin to decrease. Muscle loss also occurs due to inactivity and disease. Some people face this triple whammy and by the time the reach their 50s or 60s they are already in severe physical decline. The good news is that this trend can be reversed at any age at least to some degree. Sedentary geriatrics in nursing home have been given a weight lifting program with light hand weights and have shown marked improvement in muscle mass and strength in just a short period of time. The right nutrients can also add to the effects. Certain amino acids will help build muscle by releasing the natural Growth Hormone which is stored in the brain.
Skeletal muscle loss: cachexia, sarcopenia, and inactivity -- Evans 91 (4): 1123S -- American Journal of Clinical Nutrition