Take Charles Barkley.
As a HS junior, he was 5'10" and 220 lbs and couldn't make the varsity team. He did continue to work with his skills which made him one of the most versatile power forwards ever, but when he shot up to 6'5" by senior year, he was a budding superstar.
I subscribe to the 10000 hour rule, which is if you spend 10,000 hours practicing something, you will be an expert.
Before anyone starts scoffing at this arbitrary number, I did coach a 1st NBA draft pick when he was 10 years old. He had talent that no other kid possessed.
But more importantly, he was given opportunity to play and practice which a lesser player would not. He was on every all-star team and coaching clinic until the NBA.
The 12th man on the bench could have practiced by himself, but I think no child has the stamina, physical and mental, to work at his game 3-4-5 hours a day.