Just want to make one thing perfectly clear:
I had absolutely no thoughts, whatsoever, of making anything that could be labeled "child porn" by a reasonable adult. No Milkids, no Laura. I was not asked to do so, either.

I brought up the question in the first place because after looking at 2 of the suggested characters, I thought "she looks to me to be about 17".
None of the poser figures I was asked to use could be mistaken for a "child"; large breasts, curvy & developed figures. Obviously Virtually Mature Characters, capable of making virtual decisions, and joining the Virtual Military.
BUT. I have a 16 year old daughter. she gets mistaken for a college-age student on a frequent basis. Her mom had problems getting in to clubs until she hit about 30, because they thought she was using a fake ID, she still looks about 25.  my daughters best friend is 17, and could pass for 13., MAYBE 15 when she is wearing makeup.
With a real, live female you can check ID's; with a Virtual character, it is up to the person looking's judgement. The point being, unless the character is OBVIOUSLY at least 25 or so, you could potentially be in trouble.
My anonymous friend did have a lawyer look in to this; under current US law, NO Virtual image is illegal, unless you represent the image as being of "real" people, or it is judged to be "obscene" (which, by the way, brings up the question: how can Child Pornography NOT be obscene?). So, technically, it would be as "legal" to do this as any other Porn, but in reality it would be more risky as you can't prove the age of your Virtual Actors.
I could see a Outraged anti-porn crusader going to renderotica and pointing out a few of the "not obviously 25+" pictures and claiming they are obscene images of a Virtual 17 year old; as there is no birth certificate, the validity of the charge would be totally and completely up to the judge and/or jury, wouldn't it? THAT is where I keep hitting the cognitive dissonance, THAT is where I keep thinking to myself "this is really, really messed up".