Can users 13 years+ use Apple Pay?

See the following from Set up Apple Pay - Apple Support

If you're less than 13 years old, Apple Pay isn't available and you can't add a card in the Wallet app. Age varies by country or region.

Users under 13 can use Apple Cash to make purchases in stores and websites that accept Apple Pay and to send and receive money. See: Set up and use Apple Cash Family - Apple Support

Yes, under limited circumstances. The limiting factor is US state laws the limit at what age a minor can enter into a legal contract.

In almost all circumstances, minors only have a debit card or are authorized users on a credit card. Being an authorized user means the minor child has no responsibility for any charges. The account owner accepts all responsibility for any and all charges the child makes.

Are you prepared to payoff the $500 (or $1000 or higher) purchase your authorized user makes on Apple App Store on games, etc.? This is not uncommon and the posts/comments just here are littered with parents demanding Apple or their bank demanding a refund. In my experience, refunds are not granted.

What does Apple do to prevent such occurrences as outlined above? Apple Family Sharing is a good place to start, but even it has it’s limitations as to the protection it can provide.

Set up Family Sharing - Apple Support