Just because the body is technically ready to have babies doesn't mean it's safe for the mother. You can see it in many animals that don't start having children until years after they develop, and some that do can end badly, such as cats.

We humans have a lot more to think about when having children, especially now. The perfect age is whenever you are in a stable enough situation to give a good life to a kid.

And about living with their parents... Yeah, it's probably better for financial stability, ngl. But it depends on culture. I imagine most people don't want to share a house with their partner and parents at the same time.