Long bones are composed of a diaphysis (shaft) and epiphysis (rounded ends). At the bottom of the epiphysis you find the metaphysis, which contains the epiphyseal "growth" plates which are responsible for growth in the length of the bone. Throughout childhood and into adolescence, this area contains cartilage that allows such growth to occur. Over time, ossification turns the cartilage into bone. In early adulthood the cartilage stops growing and the epiphyseal plate completely ossifies so the bone can no longer grow in length.
By looking at the amount of cartilage relative to bone in the metaphyseal area, one might determine whether the skeleton was that of a child, adolescent, or adult.