^This.
I mean, you could be a genius at birth, smarter than the world combined, but you most likely have to wait till you're 18 before you can vote in the elections.
It is easier to stick to 18+, because in general those players are more mature or have other benefits (no, curfew, more accesible to raid shcedules etc)
However, if you don't mind reasoning your way in, I am sure that if you are as mature as you look, you will have no problem getting into 18+ guilds as a minor.