Healthy bone consists of a strong mesh made of protein and minerals(particularly calcium).This mesh is living tissue that is constantly being renewed by 2 types of cells. One type(osteoblasts) builds up new bone and the other(osteoclasts) breaks down old bone. Up to our mid-20s our skeleton is strengthened, but from our 40s onwards our bones gradually lose their density as a natural part of ageing.
Attribution: BUPA