IMO U-20 or U-21 are better than U-19.
U-19 depends just on random. You have to wait for the draft and than quickly search for players. NTU19-manager can´t work with NTU19, can´t build the good base for team.
On the other side, if you could have player in the team 2more seasons, NTU21-manager can better manage things around the team.
IN U-19 the random factor will rule. In U-21 NT-manager can better show his abbilities to manage NTU21.