First of all, age is only ONE factor.
What is her actual size? And what is her build? Is she slight and twiggy? Is she built like a linebacker? (Don't laugh, my g/f is).
Does she rely on fast swing speed for her power, or does she swing slowly and hit the ball flat?
What does SHE want?
Give her a list of WTA racket weights with the corresponding player weight.
FEW use rackets heavier than 11.4 oz.
If she has a fast swing speed, loopy modern strokes, a lighter racket is fine, but maybe not sub 10oz.
If she has a slow swing speed, meets the ball flat to get pace, then a heavier racket is in order.