People have infantilization fetishes - where they wear diapers and shit. You can certainly imagine someone making a cartoon of that in a sexual way.
Adults often don't look their ages. John Mulaney famously had a set about resembling a child when he was 29.
What if someone generated an image that looked like a teenager, but there was a driver's license in the frame that said the person was an adult?
Real people have ages. Imaginary people are imaginary.
What about art from the antiquity when what we would call a teenager would have been treated as an adult? Surely someone painted people having sex before "the age of consent" was a well-defined term. Is it illegal to own that painting?