Puberty is when the body goes through a storm of hormones that change us from children to adults. This video talks about infants going through their own puberty. We should probably coin a different word for this stage, because "puberty" has its own connotations. But it's true that infants experience a rush of hormones that change them in the first six months or so. Baby boys can register a level of testosterone that rival an adult male's! The purpose of these changes is different from that of teenagers, though. Babies are changing from dependent fetuses that are controlled by the mother's hormones to being their own person, and that requires some maturing of the internal organs and a regulation of the chemical production that will make them who they will eventually become.
We are amazed by watching infants grow and become aware of the world in their first year, but the effects of these hormone changes are not all that noticeable, except for their hair. Many babies actually lose hair after birth, and it's not just from lying down a lot. When it grows back in, it could be a completely different texture or color!