Because we do not want the legal liability that comes with dealing with minors, nor do we want the inevitable headaches from angry parents when their kids join.
It is entirely true that many realize they are Satanists before they are legal adults. It is also true that many of these individuals are more mature and worthy than most adults around them. But, I trust such mature young folks will understand why we request they wait until they are of age before applying. We'll still be here.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."