Actual porn sites have to have written records proving their models are over 18. If Grok wakes to generate porn, they can go ahead and prove that the models in their training images were of age, regardless of how they look. Nobody is suggesting to trust the AI (or anyone else) to determine the age of a person based solely on appearance.

Unless of course you're trying to argue that sexualized images of children generated by grok can't be proven to be images of children, because that's ridiculous.