Birds can fly because their bones are hollow. That's what I was always told growing up. Turns out your bones are hollow too, but you can't fly. Bones are hollow to keep them light. To make them strong they have an internal structure comprised of tiny struts called trabecula. As bone density decreases the struts get thinner and weaker. Doctors call this osteopenia. This is still reversible. When enough of the struts erode completely away it is called osteoporosis. You ability to repair this damage is greatly reduced.
Strength training can increase bone density, can stop the progression of osteoporosis, and it can reverse osteopenia - two notable cases where clients of New Orleans Personal Trainers and Austin TX Personal Trainers reversed their osteopenia.