There are not many top players that were not already good by 18, it's probably a lot rarer to find players that did not pick up a cue till their 20s or 30s and became great at it. Dechaine won like 4 Jr yearly events in a row (maybe the SBE one?).
Top 5? I thinks it comes down to what part of the country you are from. Agreed, most great players were already good as youngsters and could hang with some pro players. Tyler Strawn comes to mind as a very good youngster but there’s too many for a top 5 for me. Life gets in the way of many young players that were great from ever reaching their potential… family, kids, work, hard knocks. And dare I say about the game that I love, sometimes pool gets in the way of young players from ever reaching their potential in life. I admire anyone that does it well enough to make a living of it.