Haha, I eventually sold some of them. It's almost impossible to build a competitive team out of only youth players. While the ones you pay for can be in your starting lineup for a few seasons without investing greens (if you don't mind having a low overall rating like 3-4 star) the free ones are just to bad and it's pointless. Especially because I decided to keep this team in a Gold I association to farm some tokens and I just can't be a 30% team there.
It would be funny and cool if we could build a squad consisting of only youth players with the new update now. And also if all of them were from one nationality (or you could set it up like this)