Play well for an 18 YO? sure? Play well relative to the league, probably not. I like how you ambiguously word things though. I can see a 18/19 YO score ~15 points in a game in a top league, but I'd imagine the people on the court with him were good enough to cover the obvious blaring difference in skills.
The whole point of this thread is discussing whether an 18 YO trainee is overrated or not. Obviously you think so. I don't think they're overrated, but some are quite outrageously priced.
As mentioned, which you failed to address, an 18 YO will have a whole season of pops more than a 19 YO. Add on the fact that 18 is the fastest year of training. This gap keeps widening the later you start. An 18 YO will also have an extra season of playability than a 19 YO, again this gap widens the later you start.
Sure, not all of us want U21, or NT. But I play this game to win, and get to the top. I can't speak for others in this case. However, my logic is, if I want to be the best, I'll have to build the best team I can. Now how will I do that? Buying players I need, and training top players. The first part is based off the transfer list, while the latter part is based off training. If I want to train top level players then I want the best training available, regardless of U21 or NT.