Originally Posted by xFullCourtTenniSx
Also, I think you're misinterpreting (or I am). The green ball is the standard ball. You mean "should they be using red and yellow balls until they are as good as these players?"
They should use them until they become solid. Being as good as those kids with an adult or adolescent body is quite a high level of play.
There are specially made green training balls too which are one level above the red and orange balls but below regular green balls. They're not simply "practice balls" either. Just slightly lower speed than regular balls. They're two-toned in some manner with lighter and darker green markings.