No one cares if you're eighteen or not when you sign up to PayPal (same as ebay). No one checks, they just want to make sure you're mature enough and can take the responsibility if something goes wrong.
You don't need to use cards with PayPal. You can directly connect your bank account to PayPal and pay that way (as I do). Direct debit is where you directly send the money from your bank account to their bank account. In my experience, this is best done with internet banking. It makes it much easier, for everything.