During the research Piotr Unierzyski interviewed and/or tested over 1000 junior tennis players from more than 40 countries. Among them were Roger Federer, Kim Clijsters, Justine Henin, Guillermo Coria, and many other well known top players. The average age of the players was about 12 or 13 years old when they were tested or interviewed and some ...
https://www.tennisconsult.com/identify-talented-tennis-players-juniors-play