Where "good enough" wrt. correctness doesn't mean crypto-graphically pro-oven but only "you need to do some hacky things to circumvent it". Because IMHO for anything which doesn't also require to know the _exact_ identity of the user that is good enough.
Where "good enough" wrt. privacy doesn't mean perfect, but means for the "age verification provider" not worse then what fingerprinting can do even if you have a ad-blocker and for the "age verification checking" website means yes or no and that's all they get (preferable, in worst case also the identity of the "age verification provider" (e.g. some organization) id but under no circumstances anything more than that).
Where "good enough" wrt. usability means you have to use an verification provider of your country (because what adult means differs).
This solution isn't perfect, it doesn't distinguish between different countries having different ages for being adult and intentionally doesn't handle other age steps like 12+, 16+ (mainly matters for games).
Honestly the only real problem I see is that in the US it can't be used to buy alcohol because for some reasons they don't allow adult to buy alcohol until 21 (but do allow them to do decisions way more dangerous then that). Except it's not really a problem as the US can define "internet adult" to be 21+, given that this is mainly needed for porn, buying alcohol etc. the US would love doing that anyway AFIK.
I mean sometimes the perfect solutions is just to bothersome bother with it (the perfect solutions probably needs something like 0 knowledge proofs to preserve privacy).