I'm wondering if we will see a company like Square start supporting this soon with their Hardware/software for stores that use them as cash registers/card readers. I think that would help this gain traction quick. Imaging being able to walk up to the cash register and scan your Apple Watch/iPhone and it both verifies you are over 21, and pays for your goods all with one scan.
Most of the companies that already offer age verification systems already support this (here is an example from the App Store, there are many others). POS systems that read driver’s license bar codes are very likely to have NFC for ApplePay (and other Tap to Pay systems), so it would not take much work.