I don't see the point of drawing the line at a specific number. Everyone matures at a different pace, so while someone might have a high level of maturity at a young age, others not so much, obviously.
I mentioned how at a young age I'd mostly made up my mind about the fact that I'd CTB, after repeated, rigorous thinking that always led to the same conclusion, because I'm trying to demonstrate just how arbitrary the number 21 is. You can't measure emotional maturity with numbers.