I recently was asked by one of the minors who I know through my work in youth ministry as to why I had never added him as a online (Facebook) friend, especially since I had friended quite a few of his peers. Here is my response.
When I first signed up for my Facebook (and other online social networks) account, I used it as a means to connect real life to online life, with no distinction between the two. Over time, my use has changed to view social networking not as the online analog to real life but as a means of enhancing communication. With the nature of social networking being what it is, I quickly came to see the problems inherent with adding people indiscriminately, even those who were mere casual acquaintances. Furthermore, as a person who is actively involved in youth ministry service and as a board member of a not-for-profit corporation that works with minors as its primary purpose, I am sensitive to the legal implications and ramifications of such interactions with minors, especially those with whom my contact is otherwise minimal. In short, I do not want to give the wrong impression in actively seeking out online contacts with those who not of legal age unless there is more than a casual or incidental connection in our lives.
To this end, I no longer send friends requests to non-relatives under the age of majority except in rare circumstances (such as when I've made a relatively significant connection through our mutual ministry interactions or they explicitly request it) so as to avoid giving the wrong impression. I have not unfriended any persons previously added as I see no problem with the connections themselves; to do so would only cause more harm than good (the past is exactly that and what is done is done). I accept all friend requests that are initiated by the other person when I know them personally without regard to age (there being a clear record of who initiated the online connection).
Most of your Facebook friends who are mine were probably added through my request before I made this change in personal practice. If you wish to be my Facebook friend, then by all means, send me a request and I'll gladly accept it. If you know of anyone who might appreciate a friend request, send them a friend suggestion. If I know them, I'll gladly accept their friend request, too.