I dont get it, how did you get Bitcoins in the first place if you are under age? So now you cant withdraw it because no bank gives you cc but most banks would offer you a bank account provided your parents agree to it. Let's assume in your country even that doesn't happen then best option is why don't you exchange here on this forum. This will quickest and easiest way for you. So here's how it will work, you find a person to exchange then he will ask you to send and he will pay you Fiat, check their ratings some charge a fee others don't, I personally prefer this mode.
Anyone can get/earn bitcoin, there's no age limit.
Op if you aren't able to get cash, then buying gift cards is another great option, you can buy something off amazon or some other online stores.
