Most of the bones are still cartilage when a baby is born. Only some are developing into bone. When the heel starts to develop bone, there is usually a large area of growth that begins in the middle of the cartilage heel. This area of bone spreads and expands into cartilage. Ossification, another area of bone development happens at the back of the heel bone. The two areas of developing bone will have cartilage between them - this is how the bone grows in size. At about 16 years of age, the growth is nearly complete, these two bony areas merge together.