Bones are not lifeless structures. New bone is constantly being made as old bone is broken down. In the three month bone remodeling process, bone cells called osteoclasts break down the old bone, leaving tiny cavities. Osteoblasts, another type of bone cell, then line the bone with protein fibers. These fibers are then hardened by the mineral deposits, calcium and phosphorus.
Children grow rapidly because their bodies produce bone more quickly than old bone is removed.
By the time a woman is in her middle twenties she has about 98 percent of her skeletal mass. After reaching peak bone mass in her mid-thirties, new bone continues to be made. However bone mass starts to decrease as old bone is broken down faster than new bone is made, which happens as woman inch toward menopause. As more bone breaks down, it is easier to get a fracture. Even simple tasks can have a serious outcome.