Wilfred Benitez turned pro at 15 and by 17 had won the world light welterweight title beating ring legend Antonio Cervantes. Cervantes was about 31 at the time and had had well over 100 pro fights. Age is irrelevant if you've got the goods.
Very irresponsible of the coach to allow that if it's true. The 20 year old was much bigger and stronger. The coach would have had no idea how good the 20 year old was if he really did walk in off the street. The coach should be banned.
Seen this in the gym a few times. Sometimes you get a guy coming in who actually says the things that all new boxers think (yo mang I totally iced the **** out of this dude in a streetfight once it was just like mike tyson mang that's how I know I got the gift yo. No really, let me in the ring, I ain't even trained and I'll clown all these bitches anyway, I'm a natural.) It's just a personality type that some people have and it's a great opportunity for the coach to make an example for the gym. Basically saying "Look, what I teach you works. You might be getting beat up in sparring by the other guys but look what happens when a guy when never trained tries it." Fun to watch people get beat up when they deserve it lol.