What causes bones to become brittle as people age?

As people age, bones become brittle primarily due to an imbalance between bone formation and bone resorption. Specialized cells called osteoblasts build new bone tissue, while osteoclasts break down old bone. In younger people these processes stay balanced, but with age osteoclast activity increasingly outpaces osteoblast activity, leading to a net loss of bone density. Hormonal changes play a major role, particularly the decline of estrogen in women after menopause and testosterone in men, since both hormones help maintain bone mass. Reduced calcium absorption in the gut, lower vitamin D levels, decreased physical activity, and diminished production of growth hormone also contribute. The result is osteoporosis, where bones become porous and fragile and fracture more easily.