During aging, cells that build new bone tissue become less active, while the activity of cells that break it down increases. This leads to a loss of bone mass beginning around middle age, making bones more fragile. This can eventually lead to osteoporosis, a severe weakening of the bones that is associated with more frequent fractures and therefore potentially chronic pain.
Within the joints, cartilage that protects the ends of the two bones can degenerate with age. The loss of this tissue exposes the bone beneath, which begins to be eroded as well, leading to osteoarthritis.
Bone aging is worsened by lack of exercise (exercise stimulates the cells that build bone tissue), and by lack of certain nutrients, especially calcium and vitamin D.