Osteoporosis is a condition where bones become weak and brittle and break easily. It most commonly affects post-menopausal women. Bones reach their maximum strength in one's 20s and 30s, but then begin losing density after age 35 as more bone is lost than replaced. Risk factors include age, sex, menopause, diet, family history, smoking, alcohol, and lack of exercise. Weight-bearing exercises are important for building strong bones and reducing osteoporosis risk by maximizing peak bone density when young.