Killy always seemed about the same age to me. I think it's just the evolution of Nihei's art style. His faces start out elongated, minimalistic, and use very thin lines. Later, the lines are thicker, the faces not quite as stretched and anime like, and he uses thincker strokes more often.

As for emotion, I think Killy definately has emotion. It's just use very sparingly because of the way Nihei seems to try and emphasize the monotony and emptiness of the megastructure. The characters are detatched, but still human. Also, it seems to me that Killy is a cyborg or an android, rather than a mindless safeguard machine or an almost alien silicon creature. He's like Dhomochevsky and Sanakan, but he was created to save humanity, and thus to be more compassionate.

I think it was shortly after when killy stopped eating that he started becoming a stone faced killing machine

I mean... would you be happy if you were hungry?