The coaches are in the opinion that certain players learn more and develop more effectively playing up an age group. Mather is one example where he has basically skipped U/18 football and is now with the U/23's. Quite a few players do this. Greenwood played up a level, Shoretire sometimes 2/3 levels.
I was watching us lose to Chelsea quite heavily a few years back when we had the likes of Burke, Veevers, Kavanagh, Hampson, Murray and others. Technically we were just as good but physically we couldn't compete...we were tiny in comparison. But those players develop a range of different skills and mindsets by doing that. As long as you don't lose every week (which we don't)...then hopefully it supports the pathway through to the first team.
Taking on smaller more technical players has always been part of our approach, but playing quite a few players up a group seems to be a more recent strategy.