Currently it is explicitly against the law[0]:

  (a)In General.—Any person who, in a circumstance described in subsection (d), knowingly produces, distributes, receives, or possesses with intent to distribute, a visual depiction of any kind, including a drawing, cartoon, sculpture, or painting, that—
    (1)
      (A)depicts a minor engaging in sexually explicit conduct; and
      (B)is obscene; or ...
  (b)...
  (c)Nonrequired Element of Offense.—
  It is not a required element of any offense under this section that the minor depicted actually exist.

[0]https://www.law.cornell.edu/uscode/text/18/1466A

I wonder how you'd actually go about prosecuting that. What's the line between crass and illegal?

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?