The difference between the number of bones in an adult and an infant has to due with bone development. Some bones, such as the cranium, are fragmented in infants. At birth, the cranium is divided into three unfused plates that form together later in life. Additionally, there is the process of ossification. Through this process, cartilage turns into bone over a period in which cells and arteries develop. All bones begin as cartilage, and many are still in that state at birth. Bone growth remains accelerated until about age 20. This is why bones in infants and children are able to heal faster than in adults.
A fully developed human body has two skeletons, the axial and the appendicular. The axial skeleton is composed of the skull and torso. The appendicular skeleton is composed of the remaining bones from both the upper and lower extremities. The appendicular skeleton includes 120 bones, all of which come in pairs. The axial skeleton includes many unpaired bones. Just 46 of the 80 bones in this skeleton are paired.Learn more about Bones