It depends when they start. If they started young they will get the needed training in early and can concentrate on things like strategy or fitness or adding a new shot etc.
There is a theory that most pursuits take 10,000 hours to master. So if they kid started when he was 8..and played 1200 hours each year he could start to play less by the time he was 16. But keep in mind he had to play several ours a day to hit that 1200 a year number.
Agassi for example could probably goof off some and still play great tennis. That gets chalked up to talent but its probably his dad making him play alot when he was tiny..