All devices with Touch ID (except the iPhone 5s) support Apple Pay, but not every iOS device can make in-store purchases. Only devices with an NFC chip can use the Apple Pay terminals at retail locations.
Supported Apple Pay Devices:
iPhone 6 Plus
iPad mini 3
iPad Air 2
Only the iPhone 6 and 6 Plus or Apple Watch paired with an iPhone 5 or later can make in-store purchases. Devices without an NFC chip (iPad mini 3, iPad Air 2) can only make online or in-app purchases using Apple Pay and Touch ID.