No age limit but capacity limit.
When I say capacity, I think that the person must prove, via a quizz by example or another test, that he (or she) knows the rights and the obligations attached to a credit card. So, a person who is major will not have necessary have the right to have a credit card. The consummer must be protected.
Credit cards providers would be have the obligation to maintain and finance a site with all the decisions of the jurisprudence in this domain and communicate this information with the card if they decide to give it.