Because of the way the laws were written (and overturned, then rewritten) pornography depicting children is illegal, however Pornography depicting adults posing as children is not, so long as they put the proper disclaimer stating that they are in fact of age in the porn. (Which is why it is legal for say, a thirty year old porn-star to make a video where she is a high-school freshman fucking her gym teacher to make the cheer-squad).

Now, legally cartoons do not inherently have age. even if they look like they do. so for the same reason that a young-looking porn-star can still legally create porn so long as her actual age is above 18, porn can be created of child-like characters so long as they state that they are above 18. (since fictional characters are, well, fictional, their age is entirely dependent on what the author says it is. if the author says this version of Star is fricking forty then she is just a very very young looking forty year old).

Of course, functionally that doesn't actually matter if anyone cares about it, since it could still be considered illegal under obscenity laws, because obscenity laws are bullshit. but in practice most people have better things to do then worry about people jacking it to cartoons.