Yeah my visa card is not linking either, you are not alone.
My visa works fine. I use a debit instead of a credit.
Are you allowed to use Coinbase and is Coinbase aware of your age ? Well, maybe no one here knows your actual age.
Haha, Coinbase knows my age and they dont care. I even got emailed by the CEO! Part of the message was saying that I was doing well for such a young age.