It's a short life.
All of us start out as young infants and grow up to be adults.
All children are not innocent though. Neither are adults.

Lots of adults act like children and I've seen young adults act more mature than elderly people.

You mention a lot of the time of the innocence of children. That may be true from the beginning of their life, but that can be taken away in many different ways. But usually it's from the parents not doing their job right.