If someone takes an underaged head and photoshops it onto a naked, 18 year old's body, is it technically child pornography?

It could be. The Supreme Court has held that a computer generated image of a child engaging in sex acts is chid pornography. This is even true if the computer generated graphics were not modeled after a real person.

In short, it is child pornography anytime you have a picture that APPEARS to have a minor engaging in sexual acts. So if the picture referred to in the question appears to be of a minor, then it is illegal.

Guys, I would NOT do this. I'm asking as a hypothetical! I think to do this would be bad. I can't stress that enough!

There was an earlier question about Emma Watson, and someone having possibly photoshopped her face prior to 15 on what probably was an older nude body. I wanted to know the legal status.

I'm not a pervert!

This is in response to what happened with Emma Watson and that question wasn't it? Sorry you got DR'd for something that you wouldn't do. I can't tell you for sure. I would think it is some criminal act, even if not full blown pornography.

I don't know how this question can have -6 rating with no answers yet? If the question offends you, just move on if you can't handle using your mind for all questions. But in answer to your question, YES it IS considered child pornagraphy if you cut and paste a minor's head/face to any nude or adult themed photos OR drawings. And for good reason! These images feed the SICK proclivities of the worst kind of societal parasite.

Yup. Read the U.S. Code Title 18 Section 2256, part C:

“child pornography” means any visual depiction, including any photograph, film, video, picture, or computer or computer-generated image or picture, whether made or produced by electronic, mechanical, or other means, of sexually explicit conduct, where—

(A) the production of such visual depiction involves the use of a minor engaging in sexually explicit conduct;
(B) such visual depiction is a digital image, computer image, or computer-generated image that is, or is indistinguishable from, that of a minor engaging in sexually explicit conduct; or
(C) such visual depiction has been created, adapted, or modified to appear that an identifiable minor is engaging in sexually explicit conduct.

An “identifiable minor”— means a person who was a minor at the time the visual depiction was created, adapted, or modified; or whose image as a minor was used in creating, adapting, or modifying the visual depiction; and who is recognizable as an actual person by the person’s face, likeness, or other distinguishing characteristic, such as a unique birthmark or other recognizable feature;

I don't know, but I wouldn't take the chance on doing it, it isn't worth the risk.