It's not about the age, but whole identity. You know you are serving ads to a real person and not a bot and so on and you can correlate person across different services with 100% accuracy. Currently you can still reasonably easy fake a persona.
Something like https://protocol.humanidentity.io (disclaimer: I built it, sorry for the plug) or any other privacy preserving service might work better. A platform can then require that a person verifies age in a privacy preserving way before viewing adult content.
Or perhaps golfing at the right clubs to make it a defacto industry standard like ID.me seems poised to become?
I hate seeing stuff like this once and then never again due to people who are capable of making something this… Good being unable to “play the game” or whatever optimize to break the social-moral glass ceiling for a given problem space.