There was a study done in Poland taking over 1000 junior tennis players from over 40 countries and tracking them from 1994 to 2002. The average age was 12 to 13 years old. What they found was the better players as adults were smaller than average for their age, were less powerful, were faster and more agile, started playing tournaments at age ...
https://lifetimeperformancepbg.com/how-much-is-enough-the-junior-tennis-player-dilemma