Actually after some testing, some young players can perform very well on their 1st year. for example, my DL, Nic Brannan (got it from auction), scored 10/10 in his debut (18 years old) but several others can only score 6 or max 8. For example no matter how many his stars are (7 at the moment), my power trained MC, Linderd Hassanudin, which I bought from Youth Academy, rarely scores 8 in any game.

So some U-21 players can perform well but mostly they are just average due to lack of experience