I suspect that if u21 NT matches will not give additional xp as big NT-s, only motivator for managers would be possible trophy of u21 world/continental champion. Currently remarkable amount of managers want to guide NT just for boosting their own player(for me there is some barrier to play my own player in NT, so I do it only when I'm sure he is worth it). Maybe someone not enough brave to start from any big NT can get some courage after u21 NT manager career, but we have always plenty of unwanted countries that are happy if just someone normal will pick it.
In real life there are several series of youth championships, but mostly we don't know that player X has been youth world champion. In Europe they are mostly from southern countries as their youngsters are more acelerants comparing to northern youngsters. Meanwhile many u18 champions are later nobody in domestic level, once I looked for a Spanish youth team, where many of them spent their later career mostly in Segunda or Segunda B without any Primera experience...
Quite surely there would be very goalful matches as typical u21 keepers are around 7-8* but fieldplayers 10+*. It would also force managers to train earlier necessary attributes to their youth stars, at least couple of seasons ago half of loan-out players I checked for my second team had no trained attributes at ages 21-23.
I don't see room for different youth classes like u17 and u19 because smaller countries have no enough decent players then. Several big NT-s have problems with some positions, having no decent keeper, winger , DM etc. For example Austrian NT has more than half of players just from 3 local clubs who concentrate more for Austrian players as they have better facilities. Result: mid-ranked league in Europe and one of weakest NT-s in whole RS.
This is my quite neutral view to youth NT topic: it would be interesting, but I don't think it "must be" and I see problems with that.