A newborn baby has roughly 300 bones. The majority of the "bones" are cartilaginous in nature. This is necessary during fetal development and the birth process to ensure for safe development and delivery during birth a confined space. As the infant develops into a child, then adolescent, and finally an adult, the bones and cartilage continue to fuse and harden into mature skeletal bone. Examples of fusion include the sacrum, coccyx, and sternum. The bones of the skull also harden and fuse into one structure. Final hardening and fusion doesn't occur until eighteen (18) to twenty-five (25) years of age. This is to allow the structures, or organs, in which he bones protect to grow and develop.
Many people disagree on how many bones a newborn baby has. This is because:
1) Some "bones" are not entirely fused yet and will later become one bone. For instance, babies are born with their skulls separated into three parts so that their heads can fit through the birth canal during labor. As they grow, these bones will fuse together to form one whole skull.
2) Many of what will later become bones are actually only cartilage (called cartilaginous bones) when a baby is born. Again, this allows for the body to fit through the birth canal, but is also because when born babies have not fully completed their fetal development. This cartilage will calcify into bone as they grow.
Thus, the number of bones a newborn has can vary from source to source as some consider the cartilage bone and some consider each piece of a separated bone structure to be an individual bone. However, general consensus agrees that a newborn baby has about 300 bones.
Adults have fewer bones than newborns because of the calcification process of turning cartilage into bone and because of fusion. As you age, many of your bones will start to fuse together and become one bone. For instance, your skull fuses together into one piece. Similarly, many vertebra in your spine will fuse together later in life. This is why adults only have 206 bones while babies have about 100 more.