Muscle cells are replaced by fat as we age & nerve
pathways that aren't being stimulated die out.
Make sure that the weakness is not being caused by other conditions, however. It's too easy to say, "you're just getting old." conditions that affect muscles, nerves, and energy can be obstacles. See "strong women stay young" (book) and "23 and1/2 hours" (http://www.Youtube.Com/watch?V=auains6higo).