I know why. It's the fatigues. When you see fatigues and a buzz cut, you think soldier, and soldiers are adults. If the kid had been photographed in a pr. of Lightning McQueen pjs the photo would be giving a very different vibe.
Also, the woman's posture has something to do with it, too, I think; it's that subliminal thing. Standing that way with her upstage leg turned outward reminds me of Shirley Maclaine as Irma la Douce -- who was a hooker.