it's not just a matter of maturity.. it's a matter of a field's INSURANCE POLICY. Some fields don't have a policy that can cover minors. Others do.
That's even before you get into whether or not the players of that field will accept a minor among them. Some players don't care... kids are easier kills. :P Some can't stand playing with minors.
It's not maturity, it's not gear, it's not size, it's not age.
THEN everything else.