Your bones continue to evolve and change throughout your life. However, at about age 35 for women and slightly later for men, bone density begins to decrease. Many factors influence the processes carried on by bone tissue. Other hormones aid in the release of calcium and the breakdown of bone mass. Third, there is physical activity.
Bones respond to mechanical stress by becoming denser and stronger.
This stress comes primarily from weight-bearing activity, in which your legs support your body, and from strength building activity such as weightlifting. If you walk a lot, for example, your leg bones will respond by increasing their mass. If you regularly use your arms to lift weights or swing a tennis racket, the bones in your arms will grow stronger. Perhaps the most dramatic demonstration of the influence of weight bearing exercise is found in astronauts who spend several weeks in space. In this weightless environment, even these health young people lose significant amounts of bone mass.