i actually like this idea, but instead of going off age, why not go off salary?
Reason for the suggestion in change is that if you use Salary as a baseline, the coaches of these Veteran NT/U21's would have to make decisions based on player profiles. Where as age, if a team has a trainer with the career extension function, then they'll remain at a higher lvl for longer. So if you use Age as a deciding factor, then teams who have career extension managers would have an advantage. if it's salary it's an equalizer for everyone.
Futhermore, if you use salary, then even the smaller nations would be likely to field more competitive teams too!