Do the players keep evolving in the youth center?
Is it better if I keep a young player for a while before promoting him to first team?

In the cdf, players have to play every day to develop
AND until the age of 24 (if you don't get out) then they disappear

Mouad Zeidane: Do the players keep evolving in the youth center?
Is it better if I keep a young player for a while before promoting him to first team?

If a player leaves the training centre late (at 22, for example), he will have a better overall grade and better stamina than if you take him out early.
If you take him out early, you'll have more work to do to improve him, but on the other hand, with the right amount of playing time and assiduous daily training, you'll be able to turn him into a better player.