The younger just finished a seven-year study begun at Ohio State. The ultimate objective is to eliminate the threat of osteoporosis, now more prevalent as people live longer. Women are at greatest risk, four times more susceptible than men, for whom it is generally only an issue in the very elderly. Usually problems don't show up until the later years, after menopause. We also know now that taking calcium supplements and exercising at age 60 can still make a difference.
Bone is a live tissue, in a constant process of formation and degradation.
That's the whole key, to slow the process, so you won't reach the fracture threshold.