These ideas are all fundamentally misguided. Let’s take a step back what we are trying to do here: We want to create a system so that the government can withhold certain information from certain people. That’s both difficult and dangerous.
PornHub’s idea requires cooperation from the hosters. You are not likely to get global agreement on that. So you will still need to do something about those foreign sites, such as blocking them.
At that point, such a law would achieve 2 things:
Blocklists that parents can install on their devices already exist, so there would be no change in that regard.
Of course, minors have no trouble circumventing such software. They have plenty of time and they are horny. You can’t win. The only faint hope might be to include such features at deeper levels, similar to existing DRM schemes. This would be ripe for abuse by bad actors or governments. It certainly would be used against the consumer by the copyright industry and tech monopolies; just like existing DRM schemes.
So we really should ask why we would want to walk further down this expensive, hostile, and dangerous path. Are we afraid that masturbation causes blindness?
Rich people have staff that knows all their secrets; assistants, bodyguards, maids, drivers, … Their staff might abuse their position to steal, blackmail, or even sign book deals. They still have the staff.
When AI agents become useful for normal people, normal people will use them.