To me it's the face that makes her look like a teenager, not the body. Particularly her eyes, believe it or not. But on further examination, I think her nose looks very weird. Just a little too short sticking out from her face to be human, but her eyes are distractingly pretty, so I didn't notice that at first.
Yeah, that nose really messes up a pretty picture and makes it look like she's actually an alien pretending to be human. Otherwise you did her so masterfully for a speedpainting, for sure.