I am 15 but still cannot add card to apple pay.

Thank you for your reply, and for checking carefully.

Seems age for starting Apple Pay can vary by country or region.

or

Is it possible there are any parental or other limiting controls set:

Eg: Family Sharing / Ask to Buy / Screen Time

or

See if anything here helps:

If you can't add a card to Apple Wallet to use with Apple Pay - Apple Support

Hi Richard,

Welcome to the Apple Community

You might want to check the birthdate listed in your Apple ID account.

iPhone: Go to Settings > [your name].

Tap Name, Phone Numbers, Email. You might need to enter your Apple ID password

Tap Birthday

Apple Pay may not be supported in all countries, or by all card-providers.

Countries and regions that support Apple Pay – Apple Support

See Apple Support's article:

How to Set up Apple Pay - Apple Support

Hope this helps :-)