I have to say that without parental accompaniment, a coven shouldn't really initiate anyone under 18. Not to say that youngsters who are truly interested in witchcraft shouldn't be encouraged, and supported. Another possibility is for a coven to sponsor and moderate a 'sub coven,' if you will, for those younger Wiccans. The parents may suspect sexual relations or sexual abuse. For robed covens, there should be a very good reason why underage members aren't allowed.
Minors have freedom of religion too.
Any kind of religion would help minors in personal development and age should not be the reason not to choose paganism. Ideally it would be wonderful if all parents were accepting of their children's religious choice and had enough faith in them to let them freely believe as they wished. In reality, however, I know from experience that this is not always the case. As a coven leader, I am approached periodically by young people wishing to learn. If they are serious minded and we have permission from their parents, we allow them to attend.