I sometimes have to remind myself that no matter how adult a teen may seem, in the eyes of the law they are still a minor. And that brings me to my first reason for not teaching someone under 18. And that reason is first and foremost legalities. I am in the United States of America. For instance, you could not teach a child to shoot a gun without their parents' permission.
Religion is the same.
And like some of you, I was raised in a Christian religion. I was raised Episcopal complete with catechism and First Communion. I thought other religions meant Jewish, Catholic, Methodist or Baptist. That was pretty much the extent of my base of knowledge. Had I known about Wicca, my mother would have been horrified to learn I was studying it.