Final answer:
Bones vary with age due to the process of skeletal maturity, where growth in bone length stops by age 18 to 25. Bone remodeling allows adaptation to stress and maintains bone mass and density, but density generally decreases with age, especially rapidly post-menopause in women. Bone density typically peaks around age 30, after which there is usually a decline.
Step-by-step explanation:
Why Do Bones Vary With Age?
During fetal development, childhood, and adolescence, bones become increasingly ossified and grow in size. However, once skeletal maturity is reached, typically by age 18 to 25, bones no longer grow in length. What changes over time is the bone mass and density. In the late teens or early twenties, after all the cartilage is replaced by bone and fusion at epiphyseal plates occurs, additional growth in bone length is not possible.
Bone remodeling is a continuous process that allows bones to adapt to stresses. As a result, bones can increase in thickness in response to changes such as enhanced muscle activity due to weight training. Bones that are not subjected to normal stress can lose mass, such as when a limb is immobilized in a cast. Similarly, changes in muscle strength, body weight, diet, or hormone levels, especially estrogen post-menopause in individuals with XX chromosomes, cause variations in bone thickness and strength. Bone density typically peaks at about 30 years of age and usually decreases after that, with women losing bone mass more rapidly than men.
Bones continue to be dynamic and living tissues throughout an individual's life, with the entire human skeleton being replaced roughly every 7-10 years through the process of bone remodeling. This process, which also involves mineral homeostasis, shapes the bones, repairs damage, and adapts the skeleton to different levels of physical activity and weight.