Loss in muscle mass that occurs with aging is called sarcopenia. Muscle mass is lost at a rate of four to six percent every 10 years. This starts at age 40 in women and age 60 in men.

The greatest decline in both men and women occurs with inactivity, acute illness, and after age 70. A healthy older adult has 25 percent less muscle mass than a 25 year old. At all ages, women appear to be more vulnerable to loss of lean tissue than men. However, both men and women can maintain muscle strength well into the eighth decade of life through exercise.
 
It remains uncertain how much muscle loss is due to aging and how much is caused by poor nutrition, hormone changes, and less physical activity.