I don't think this is a safe assumption.
As a counter, actual age verification is being rolled out in other places. I really don't think we're in a position of choosing between no age-based access mechanism, and age-based access mechanism. Between the anti-porn types and public demand for some kind of regulations on social media, regulation of some kind is inevitable.
Our actual choice may only be what type of restriction we can live with, and I much prefer this type to the kind that requires websites to demand my id and photos of my face. Especially since some implementations of this concept (the California one, I think) declare that websites aren't legally required to look deeper than the attested age, which is a very nice feature.
Mind you, I don't know why the legislators are bothering mandating OS support for these features. It would be much easier to mandate that websites support the feature, make it clear to them that supporting the feature appropriately will free them from liability for children accessing content, and then wait as users demand that their OS support the feature.
> I really don't think we're in a position of choosing between no age-based access mechanism, and age-based access mechanism.
It is impossible to win a battle you stop fighting.
Re: your last paragraph
I actually agree with you, if you mandate that the site has to look for an affirmative signal and if it doesn't get one, has to assume the user is the youngest possible age group. Users would demand the proper support for it.
Although it would have to have some teeth capable of biting the client software companies, because for instance, if browser(s) chose to on their own simply send "I'm over 21" to every site this becomes a pointless exercise and that applies whether the browser makers do it out of frustration that the OS support hasn't landed, or out of malice (imagine a browser that misreported age on purpose, specifically targeted at kids who want to bypass the parental controls).
Honestly though - because kids (especially the younger set) are hard pressed to buy their own hardware, a property that can only be set up out of the box, and can only be undone by using the account password of the parent who set it up, it is the perfect level of security here. And as for browsers, all you need is the gatekeepers (Apple, Google, MS) to agree not to ship in their "stores" browsers designed to evade it. Yes, you can totally compile your own browser, but most kids are using locked platforms like iOS and Android, and are by default denied permissions to run arbitrary software on platforms like Mac and Windows, so that's fine.