You need to have activated your U21-squad before you can play under 21 matches. To do this, you need a youth center of level 3 or better, and you need at least 10 players in the U21 category.
If your youth center is level 3 or better you'll have a tab "U21 squad" where you can activate or disband your U21 squad. You can also see there how many U21 players you have. These players can be in your reserve or main squad.
To be eligible for U21 matches, a player must be 21 or younger at the start of the current season. If he can play U21 at the start of the season he'll be able to play in those matches the entire season.
When you click "players" in the menu (under team) you can see which players are U21 by the tag "U21" behind their name.
Players do not need to be in the main squad to play matches. You can select your reserve squad players for U21 matches. You can't select players that are still in the youth center, so you need at least to promote them to the reserves squad.
