Why Do Adults Have Fewer Bones Than Babies?

Yes, adults have on average 206 bones and babies tend to have 275 bones. As babies grow some of their bones fuse and grow together, which is why we have fewer when we reach adulthood. This is why a baby's body is delicate, the bones need the time and space to grow, without damage which could halt this process.
Bones range a great deal in size, from the thigh bone which is on average 51 cms long (about 20 inches) down to the pisiform which is a little bone that lies at the base of the little finger. This bone is so small that it is about the same shape and size of a split pea.
Some bones are very loosely connected, such as the shoulders, knees and elbows. Others such as the pelvis and skull form tight protective casings, so that they organs inside are protected and cannot easily be harmed.

Adults have fewer bones than babies because some of the bones that you have as a baby becomes fused when you become an adult.  For example, the skull of a baby is soft and comprised of different segments that become fused after birth and the baby grows up (I am not exactly sure how long).

Babies have more bones because you start with lots of bones and when you get older they join together so you end up having lesser bones

Yes, and as the baby grows, the bones fuse together and get harder.

I don't think so but I know that Babies have fewer bones because they didn't devolop anything yet!