Babies are born with 300 or more bones and adults have 206, because the bones have fused together. The bones that fuse (in general order of fusion) are those of the: Skull, Elbow, Hip, Ankle, Knee, Wrist, and Shoulder. Some of a baby's bones are made entirely of cartilage. Other bones in a baby are partly made of cartilage. This cartilage is soft and flexible. During childhood growing, the cartilage grows and is slowly replaced by bone, with help from calcium. It may take until between ages of 18 to 25 for all bones to completely fuse.