That's really not important though is it. If you're being prosecuted (at least in the UK) for such crimes then the age of the children and the level of the pornography is assessed and taken in to account in your sentencing. If the only image you'd acquired was a nip-slip of a teenager -- or you had a selfie with a same-age partner -- then the sentence would be entirely different to having thousands of images of pre-teen rape. Whilst USA law is often ridiculous it doesn't seem to be that lame that such differentiations aren't being made.

Where are you trying to go with this rhetorical question?