It depends where you live, but most places let you get hormone blockers, which help a lot for mtf puberty since testosterone's effects are a lot more difficult to reverse than estrogen's. I'm 15 and I just started hormones so I know in some states you can get hrt from 13 I think. If it's just a matter of being allowed hormones legally, you can look up local legal requirements online. It very much depends on genetics on how you will grow when on hormones, so look at the women in your family.