I have to imagine porn sites don’t want an underage audience either.
So I think they would voluntarily state that their content is adult only.
This isn’t rocket science. We’ve rated content for decades. We simply can’t be obtuse about this and throw our hands up and do nothing because we can’t tell if obvious things like porn are adult only content the way we’ve done on every other medium.
There's no way everybody will agree what constitutes "adult only content," therefore there's no way to come up with a rating for websites.