The fact that a particular person in a pornographic video is underage is up to the government to proof if they want to convict you, say, of a child pornography

The fact that a particular person in a pornographic video is underage is up to the government to proof if they want to convict you, say, of a child pornography charge. It does not matter where the material was produced, or what kind of records they kept. Now, the government may create laws that oblige a distributor of pornography to be in possession of such records, and someone how is not may be in violation of those laws. It also doesn't matter if the material is advertised as "teen" or not. Similarly, you may argue that a site like Backpage should be legal, as long as they verify the age of the advertisers. It's a rather strange argument, after all, they can't verify that the person they verify is the person that turns up at the appointment. But that is not the law. The government is instead alleging that there is a law that makes Backpage-like advertising of prostitution illegal, regardless of whether the ages were verified or not.