I found some info here: https://ageverification.dev/

But that's difficult to read, very technical. And mostly written from the user perspective. It looks to me like they're (for once) trying to come up with a proper solution. Everyone can be an Attestation Provider, Relying Party or repurpose the white-label App. At least in theory. It's all specified and in the open. And then the European Union contributes some list of trustworthy Attestation Providers (governments, banks, mobile network providers...)

I think due to the project structure, it'll be more like the Covid-Certificate App, which could be customized by every member state and it's theoretically possible to use it as one uniform solution.

So unless there's some certification for "Relying Parties" which I missed while skimming the documentation, I'd say in theory it'd be possible to use it on a technical level. Of course it's still a preview so the EU has lots of opportunity left to mess it up.