Junior rackets are sized according to the height of the child.

Here are the guidelines:

3’ 11’’ and under = size 21 racket
3’ 11’’ to 4’ 6’’ = size 23 racket
4’ 6’’ to 5’ = size 25 or 26 racket
5’ and above = adult racket
Junior rackets are not designed for restringing, but can be re-gripped. Adult rackets can be restrung and re-gripped.

Still not sure?

Here are links to two helpful articles that discuss the factors that you should consider when you think about buying a new tennis racket:

http://protennistips.net/how-to-choose-a-tennis-racquet/

http://www.midwestsports.com/helpracquets/a/helpracquets/