Ironically, that answer would probably be more acceptable than any other. It would make perfect sense. (The Soldier often wishes it could reply to questions like that with "functional", but that tends to make people demand other responses.)

Even more ironically, being told he's "fine, thank you" makes the Soldier think the boy-slash-young-man is very much not fine.

So it tilts its head, giving him a steady look, and answers the question, "You." And, because most people would want an explanation, it adds, "You're younger than you look. And you hide and don't talk to people." Which is not really much of an explanation, honestly. But it could be construed as "I am worried about you" or "I am wary of you" or even just "I am confused by you". Given the completely bland affect the Soldier has on now that they're talking, it's hard to tell.