Bone anatomy & physiology :

A majority of our bone mass develops in adolescence. Multiple factors can influence our bone density as we age including diet, physical activity, and gender. 

The human body has multiple physiological processes that maintain our bones. In the "repair" phase, specialized cells called osteoclasts degrade bone with damaged surfaces. The "remodeling" phase follows this process, aiding in the formation of new bone. In healthy adults, this cycle aids in the maintenance of a healthy bones. Factors including aging, however, can disrupt this process leading to bone mineral loss and bone deterioration.