Devil's advocate mode:

Extending childhood is related to spending a lot of time preparing children to do things other than what they grow up with. This is necessary for a resilient society that is something more than a collection of family farms and for social mobility.

This extended "training time" is less necessary if A) you live in a preindustrialized society where you aren't expected to spend a significant amount of your life working for an employer, and B) all you are going to do is what your parents are doing on the same property they live on.