What distinguish adult humans from babies?

The answer to this is there maturity.

Babies have more bones than adults because they are growing. Once you are an adult all your bones have grown and formed together in the appropriate places.

After a human reaches a certain age, it’s considered adult hood, once you’re an adult you no longer grow taller.

When you are a baby all you do is grow. So those are som distinguishing factors, although there are many more.