After all, any and all age checks we have nowadays are a black box anyways

This is the only part I disagree with. Age verification is typically done via services like ID.me, Lexis Nexus, etc which do it via identity verification with documentation. The alternative method that most social sites have gone with is age prediction from a face scan, of which providers are more than happy to tout how they do it as differentiators. For the latter, there are even FOSS options.