It seems what would be more productive would be for sites to publish metadata about the nature of the content. Then the access method could decide whether, or which of it to show.
Analogy: parent seeing that a movie is R rated and not letting their kid see it.
Age verification means that the content origin still has to rate its material for age suitability, and match that to the user. Asking them to do just one out of these two things is easier.