An age verification requirement might stop your 12-year-old from accessing a porn site headquartered or hosted in the US, but it will do nothing to keep your kid from finding porn on any of the thousands (tens of thousands? more?) of websites hosted in various other countries who don't care about this sort of thing.
These sites are (or will be, if US-based sites become inaccessible) just as easy to find, and just as hard to block with normal parental-controls style content blocking.
Requiring age verification in the US doesn't solve the problem. It just stifles free speech and turns us even more into a Christian nanny-state. The people pushing these laws don't care about children, in reality. They care about banning pornography in the US, and this is one step on that road.
> If you believe it's hard to do it without satisfying privacy concerns, totally true!
That's not the issue. The issue is that it's impossible to achieve the stated goal (making it impossible or even hard for children to access adult content), period. Whether or not the age-verification is done in a privacy-preserving manner is irrelevant.
There are two ways to "solve" this problem. One is better parental controls, but this will always be a cat-and-mouse game, and will never be perfect. The other is to accept that your kids will sometimes see things that you don't want them to see. That's how the world has always worked, and will continue to always work. Be there for them to provide context and support when they run across these things by accident and are confused or upset, and punish when they seek it out against the rules and boundaries you've set for them. You know... be a parent, and parent them.