We have a younger u18 side than City. At least when it comes to giving minutes to 16 year olds. Not sure about average age, with players like Williams still playing.
If I recall, we gave 6 players 400+ minutes to 16 year olds. City gave 3 players aged 16, 400+ minutes. Both had similar amount of 16 year olds playing, but United had more given heavy minutes. And that's with Shea Lacey and Ibragimov having been injured, who would have surely racked up a couple more 100+ minutes. I don't think there is anybody with as many minutes as Fitzgerald for example.
The u19 squad suffered for two reasons. The more talented players are u17 players like Fitzgerald, but are still super young to really be dominating the u19 and we had injuries to players like Lacey. Given the squad make up, that was a big blow, but it's not like they were performing all that well with him either. I would expect a better u19 performance next season, if United manage to qualify via winning the u18 league.