Gabe Mirkin, M.D.
A study from the University of Maryland shows that older men and women can enlarge and strengthen muscles in the same way that younger people do.
The stimulus to make muscles stronger is to exercise them against increasing resistance. When you push against great resistance, you feel a burning that is a sign of muscle damage. When the muscle heals from that damage, it is stronger than before. You feel sore on the next day and should not exercise that muscle against heavy resistance until the soreness goes away.
Most young people can lift very heavy weights and continue to lift through the soreness, but older people should stop lifting as soon as they feel the soreness during a workout. Younger people recover faster so they can lift heavy weights every second or third days, but older people take longer for the soreness to go away, so they may be able to lift heavy only once every week or two.