I am a Russian proxy site, I make requests for you without the header. I serve you the content because I don't care about following American laws.

Alternatively, just use an older browser that doesn't serve the header.

If anything, you'd want the reverse. A header that serves as a disclaimer saying "I'm an adult, you can serve me anything" and then the host would only serve if the browser sends that header. And you'd have to turn it on through the settings/parental controls.

Now, this doesn't handle the proxy situation. You could still have a proxy site that served the request with the header for you, but there's not much you can do about that regardless.

> I am a Russian proxy site, I make requests for you without the header. I serve you the content because I don't care about following American laws.

That's no different to a law mandating identification-based age verification though. A site in a different jurisdiction can ignore that just the same.