• Maturity: He/She must be 16 - Before that I don't think kids can understand credit cards as money, I know some grownups who don't grasp the concept.
• With responsibilities comes privileges: He/She earns the money they can spend with the credit card. It is easier to spend your parents money then your hard earned money
• Rules: Set a limit to the expense - passed the limit = revoke the credit card for the time needed to put the expense inside the limit

Those are my rules, your can be different. The only important part is stick to them.