They already support ID checks as an alternative to face scanning, if the latter proves to be untenable then it's literally a case of flipping a switch to mandate ID instead.
ID is much easier to forge, it's just a flat 2-d shape. None of the physical security features come through in images.
They can't feasibly do this in the US since many people don't have drivers licenses or passports.
Personal Identity Verification (PIV) and Common Access Card (CAC) credentials used by US government & military via NFC already work on web browsers. States should just move to digital IDs stored on smartphones, with chain of trust up through the secure element...
Most people under the driving age don’t have ID’s, at least in the US.
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The long term solution would have to be some kind of integration with a government platform where the platform doesn’t see your ID and the government doesn’t see what you are signing up for.
I don’t this will happen in the US but I can see it in more privacy responding countries.
Apple and Google may also add some kind of “child flag” parents can enable which tells websites and apps this user is a child and all age checks should immediately fail.