From Gabe Mirkins' newsletter at http://www.drmirkin.com

Dear Dr. Mirkin: I'm sixty; is it too late to strengthen my bones
with exercise?

    Every American woman, and most men, will suffer from
osteoporosis if they live long enough.   Hip bones broken by
osteoporosis do not heal, and must be replaced immediately. A
study from Australia shows that regular exercise helps to keep
bones strong and exercising into later life protects bones of older
people even more (Journal Osteoporosis International, August
2006).   An earlier study from Sweden showed that men who
were highly competitive soccer players in their youth and then
gave up active sports did not have bigger and stronger bones
and did not have fewer fractures than people who never
exercised at all. On the other hand, people who did not exercise
in their youth, but started and continued their exercise programs
into later life did have larger and stronger bones.
    A person has the strongest bones at ages 20 to 30.  After
that people lose bone continuously for the rest of their lives.  Any
activity helps keep bones strong, but exercises that put extra
pressure on specific bones offer greater benefit.  That is why
weightlifting is a much better exercise for strengthening bones
than swimming.   The bones in the arm that hold the racquet in
tennis players are much stronger than the other arm.  Pick any
sport that keeps you active and try to do it daily for the rest of
your life.  Take off only when you are tired or sick or your
muscles are sore.