The only teenager I ever used was
Paco Trevino. In order to sign him in S18 as an 18 year old IFA I had to give him ML contract to boost my offer, so he was already on my 40-man roster anyways..
BUT I only used him at the very very end of his S19 season, he was something like 73 OVR and was better than my long relievers at the time so I plugged him into a 25-man spot right for the last 3 cycles to help out in the playoffs while still preserving service time. I can't imagine he would have done much better than replacement-level in prolonged S19 regular season stint that season, he wasn't good enough yet. In S20 I would have called him up right away after 30 cycles or whatever but I have a superman pitching staff on that club and didn't have any roster spots until a guy got hurt midseason. Kind of laughable that he went 15-0 in AAA, wasted a few ML WAR i'm sure, hopefully he doesn't retire with 295 career wins or something stupid like that
Edit: We missed the playoffs in S19 by 1 game / tiebreaker so he doesn't appear on the playoff log but he does have 1 regular season ML appearance in the books