If you give teenagers cash, they can't spend it online. Thanks to KYC rules and bank policies, you have to be 18 to get anything that looks like a credit card.
Best plan for teenagers is a joint checking account (dedicated to the child's use) at your bank, with a debit card they can use for day-to-day spending. You can do allowance, etc with transfers, and most bankd will play ball as long as one person on the account (the parent) is 18.