My son

Create an Apple ID for your child - Apple Support

Family Sharing - Apple

trazayre Said:

"My son: My Apple Pay says you’re not old enough to have an Apple Pay but I’m 13 My mom told me to do this."

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With your mother's permissions, contact Apple Support to find out...

Contact Apple Support:

For Financial Assistance, contact Apple Support. These are user-to-user forums. We are not Apple.  Apple barely reads these site on their own time, as it is. As you may suspect, no one here would know the answer to this. So, contact Apple Support. The'll see what has occurred, and will run you through the fix. Here is the information for contacting Apple Support:

Apple Phone Support:

• Contact Us - Choose Locations
• Proceed from there as Necessary
• Why the Wait: Phone calls are taking a bit to go through at the moment, due do lots of calls. With the Coronavirus Pandemic, many are at home at the moment, with much time on their hands to contact Apple. So, just stay on the line, and you will get through :)

Related;

What age do I need to be to use Apple Pay… - Apple Community

There may be country- or bank-specific age restrictions in addition to the Apple Pay and Apple Pay Cash ones.

you mom told you to do what to get Apple Pay to work