As long as that 18 year old (who has yet to train) has like 3 JS/OD/PAS, he could get a 5 week injury tonight and would still make U21 if he got consistent training from that point on. But I agree it's unlikely he'll get consistent training and won't make it, which is expected since he's probably behind others anyway.
Your trainees are awesome and far from the norm. U21 players don't necessarily have to be great and multi-skilled like the players you're training for senior NT.