So then you just have to weight that against the amount of pops it would take for the 18 yo 4 ball to be as good as the 19 yo 5 baller. I doubt the difference comes out to anymore than 5-8 pops. Extremely significant if youre trying to make a U21 player, but is it all that significant if youre trying to build a team?
There are pops and there are pops. If, for example, the 19 year old was a 6'11 player with all of his guard skills at average or above and decent starting big man skills, and the 18yo was a 7'4 donkey with atrocious skills all over the place, that's a gap that'll never get made up (and that too few people would even care about). And if you're looking to build a team, you'll be far better off with a more well-rounded player trained intelligently than jumping on the donkey treadmill.
Of course, ideally you find an 18 year old with the skill and potential to be a great player - and then get the 19 year old in the second round and then possibly have two guys to build around.