People with zero experience in the field will start in an entry level position. That has nothing to do with age. It is true if you are 36, 16, or 56.
The simple fact is that 16 year olds are not children. They have nothing in common with 6 year olds, but almost everything in common with 26 year olds.
The “brain fully developed” thing is a really stupid urban myth. It has been thoroughly debunked.
In fact in many ways the teenage brain is superior to older brains. Arguably our brains decay from about age 13 on.