We're talking about a law. If you distribute pornography to someone who sent the header in that request, it would be a violation of the law. But that law doesn't have any ID requirements or privacy problems, unlike the proposed one.
> However, as we have already seen, asking nicely in the HTTP headers doesn't actually work, it may even help porn peddlers better target children.
To begin with, "targeting children" is preposterous. It assumes that they would not only not care but prefer to have children as users than adults, even though children are less likely to have access to money to pay for content/subscriptions and purposely targeting children would get them into trouble even under longstanding existing laws.
On top of that, the header isn't specifying that the user is under 18, it's specifying that the user agent is requesting not to be shown pornography. It's as likely to be set when the user is a 45 year old woman as a 14 year old boy, so using it to distinguish between them wouldn't work anyway.