There's a lot of ambiguity there. What if I accuse you of making a computer generated porn image of a model that I think looks 15 but you argue really looks 18? Who says different? Can you discern a 15-year old from an 18-year old just by looking at a photo? I can't. It's computer generated. There's no model ID on record. Are you breaking the law or not?
What will probably happen is the laws will ban all CGI porn, regardless of perceived ages.
OR
All will be legalized.
There is no middle ground................