Small addition to Phil0124's post.
This is not just Apple's requirement. It can also depend on the country you live in. In the UK I don't think you can get a debit or credit card under the age of 16.
Other countries may differ further.
As Phil says, you need a card to support Apple Pay; I am only clarifying the national legal requirements.
The only requirement, is that the Apple ID being used, have a minimum Age of 13 or over set for it.
If you are over 13, and have a supported debit or credit card, you can use Apple Pay.
Read here for details:
Set up Apple Pay - Apple Support
If you are referring more specifically to Apple Pay Cash, then you need to be 18 years or older to make use of it.
More details here:
Set up Apple Pay Cash and person to person payments - Apple Support
What age do I need to be to use Apple Pay ?
