Considering the fact that you see alot of people in WoW under the age of 18 you can assume that their parents are paying for it with their credit card, they have their own credit card (co-signed by their parents), or they are using time cards. I'd expect that time cards are the most used though.
Heh I use Cards instead of CC dont like taking my money out monthly for WOW, once I spent over and forgot I had to pay for WoW...