I can see where you are coming from. At the place where I train the adult classes are 16+. There are girls in my adult class but they are 18+. I am a big guy and sometimes I ask myself the same question. However it is important to train with different types of people. The girl needs someone to train with as well. One good thing to note is sparring with the girl will teach you control and control is a big part of martial arts. Her training with an older guy like you will teach her how to fight older stronger people and take some of the fear away form her.
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