Why do humans stop growing?
Human bones grow in length due to the effects of human growth hormone on the epiphyseal plates near the ends of the bone. Over time these two growing margins decrease in size, or width, until those special cells are either gone, or can no longer respond to hGH. At that point the body has no mechanism to increase the size of the skeleton, and growth stops. In females this tends to happen earlier than in males; approximately 2 years after the onset of puberty for females.