Bought my daughter a Honda 250EX as her first quad when she was 15. Plenty of power, not too big, and it has an auto clutch so shifting was easy.
It still requires shifting, so she learns how to ride in the proper powerband, but has no clutch for her to worry about.
She absolutely loves it, and my wife hops on it, and it has more than enough power and speed for her to. It has reverse also which is nice.