Teens with credit cards

I ran into a problem with a couple who did not have a credit card to
rent a car, only checkcards we do not take. But their under 18 son had a
credit card to use. Can't take a credit card from someone under age. By
law, under age 18 can not legally sign contracts and any contract signed
will be voided by the courts. How does a kid have a credit card and his
parents do not? Maybe it was a prepaid credit card we don't accept
either. Oh well, make somebody mad every day. Post later. Bye!