Many have been asking about the correct sizes for your youth players as it pertains to your tennis racket. Each individual is different depending on size, strength, and ability level, but mostly it follows these guidelines.
3-4 yrs old: 19" racket
4-6 yrs old: 21" racket
6-7 yrs old: 23" racket
8-14 yrs old: 25" racket
14 and Up: 27" racket (adult size)
There are changes in grip size, mainly at the 25" and 27" racket, but those are determined by comfort in having a larger handle or smaller handle.