A skeleton in progress

• In babies, the bones in the head aren't fused together. This allows the head to be flexible so it can pass through the birth canal during childbirth. The bones in the skull don’t fully fuse until ages 1 to 2.

• Children have more dense, elastic tissue (called cartilage) in their joints and other bony structures (such as the ribs). This allows the bones to continue to develop and grow as the child grows. This extra cartilage develops into bone over time. By about age  16, most extra cartilage has matured into bone.

• Children have growth plates in each long bone. A growth plate is an area of soft bone at each end of the long bones. Growth plates allow the bone to grow as the child grows. The growth plates fuse by the time a child is  14 to 18 years old.