The general rule about muscle mass is "use it or lose it."

Regular exercise can keep muscle tone and bulk at its best. Still, lost muscle mass is common with aging. Part of the reason this occurs is that there are fewer neurons (brain cells) stimulating your muscles in your later years.

A few other things can lead to lower muscle mass:

Low vitamin D
Low vitamin D can cause muscle weakness, muscle wasting, and a risk for falling. It is very common for people to not have enough vitamin D. You are particularly vulnerable to having a low vitamin D level if you don’t include a lot of dairy products (fortified with vitamin D) in your diet and if you don’t spend much time in the sun.

A lack of dietary protein
One reason older people lose some muscle mass is that it is common for older people to eat less protein in their diet.

Medical conditions and medicines
Long-term illnesses and inflammation from infections or illnesses cause muscle to break down. Prednisone and other corticosteroid drugs used to treat lung disease and inflammatory conditions cause muscle to break down.

Endocrine problems
Check with your doctor if you have lost muscle mass. Your doctor may recommend tests to make sure you have enough testosterone, thyroid hormone, and growth hormone.