from sports i know (hockey), generally the next generational player is identified when they're like 12-13 years old (earlier for Gretzky). You look at the top scorers from the Brick Tournament(9-10 year old kids play in that tournament) from 10 years ago (https://www.eliteprospects.com/league/brick-invitational/201...), 3 of the top 5 scorers were drafted in the first round, and the top goalie was Team Canada's goalie at the world juniors.

edit: went back a few more years, lots of NHLers in the top 5 in scoring in the tournament, but some years are more miss than hit.

Gretzky is well-known for saying he thinks kids should play multiple sports and avoid hockey in the summer, like he did (IIRC) - he mentioned soccer, etc.

in contrast: the sport i know best, hoops, a common pattern for generational players is for them to be late bloomers because they grow up short, developing skills and competitive toughness, then get lucky and grow a half-foot late in puberty