There are emerging options for both re-usable and interoperable age assurance.
Re-usability allows you to do one age check with one age assurance provider and then use that on any other sites that provider supports. That could be through creating an account with the provider, or the use of a digital ID App. The EU is creating an AV App that will also do this, usually drawing on national eID schemes (under eIDAS regulations).
Interoperability takes that one step further by allowing users to re-use the same check acros sites and apps even if they use a competing age assurance provider.
One solution for interoperability comes from euCONSENT. This was originally a project funded by the European Commission to develop an infrastructure for online age verification and parental consent across Europe. The AVPA was a member of the consortium appointed to deliver this project, as were a number of AVPA members, academics and other experts. You can read more about the project on its website www.euCONSENT.eu
When the project ended, the consortium members wanted to continue its mission, so formed euCONSENT ASBL, a non-profit organisation which has gone on to create AgeAware, a more modern technical solution to interoperability. This will shortly go live. You can read more about it here.
Opale.io also offers a system of interoperability using its AgeKeys.