I think that is also part of why the brick and mortars want in on ebooks. Johnny and Suzie Teenybopper can easily get giftcards for their favorite big box bookstore, then buy via the website.
As far as debit cards, I know of a lot of kids who don't have debit cards with their bank accounts, so they don't go nuts and overdraft all the time. I've had a bank account since I was like 4, but I didn't actually get a debit card until I was 19 or so.
For Paypal, you have to be 18.