How would you propose I prove to you at a reasonable certainty that I am an adult, without showing you my ID, or showing it to someone else you trust to tell the truth?
And also somehow prove that the ID I gave was not fake without the government that issued it telling you that it’s genuine?

Well, I actually could do it because I’m old enough that most of my accounts are already over 18 years old, but I don’t think requiring every new Pornhub user to wait 18 years is a reasonable solution.