Something like a U24 team would make more sense than a U19 team. At 24, most players are not making their National Team (for larger nations at least) but are very skilled players still. I'd say most large nation National Team players enter the NT scene around 26-28, so U24 would be a nice balance between the U21 NT and Senior NT. Even this idea though, would cause problems for micro nations as the senior NT would want some of the U24 players on the senior team.
I'm all for a U24 team. It would also give people an incentive to train the 7 and 8 potential players, coz at least these players have an extra 3 years to play at NT level. These players would almost never make it into the NT, and are often overlooked by trainers because of their lower potential.
A U19 team would be pretty redundant coz we know most players are training 1v1 Forwards in their first season. Even if they aren't there is also not enough time frame for any player to achieve any useful builds so it's very highly reliant on starting skills.