Bone matrix mineralization markedly increases throughout the skeleton from infancy to adolescence. From then on, bone mass growth gradually slows during adolescence until peak bone mass is reached. From adult age onwards bone only proliferates naturally in the case of fractures needing repair. The benefits of physical activity on bone density are observed especially in the first stages of life. These benefits, particularly the increase in bone mineral density, continue to diminish with time, and increases in bone mass as a result of physical activity are unlikely after the age of 50 years. Subsequently, the benefits of physical activity on bone are related to lesser bone loss, that is, the beneficial effects relate to maintenance rather than gain.