What others said, if you want a trainee and still want to compete, you have to buy a 23-24 year old and work from there. If you get someone younger, then they'll play a max of 48 minutes a week, and it'll likely be a scrimmage. Training youngsters in the NBBA really hurts their experience gains. Since they'll go multiple seasons without any real game experience and minutes.
The experience mechanic greatly complicates things. It's also why I'm paying more attention to BB right now, I'm able to train youngsters in Utopia and they're getting real minutes.