Every few months we get a submission from a young adult who wrote a fantastic manuscript. It pains us to have to turn it down when we realize they are under 19.
Why do we not accept them? Because it is a nightmare. We would need to make sure the royalties go into a trust fund administered by someone other than their family and make sure they do not steal the money out from under them. They also need guardians to sign contracts and that is where everything usually falls apart.
Yes, we have seen about a dozen submissions from age 11 to 19. Yes, they can write novels much better that adults. Yes, I wish we had a legal department large enough to take care of them.