"Um... S-sixteen?" He did, of course, know how old he was, but the hesitancy and questioning tone was born of wondering why she wanted to know. It made him self-conscious, because people usually only asked when they were preparing to insult him.
Hunching his shoulders certainly did little to make him look his age; try as he might, he seemed to be hovering between a man and a boy -- and very, very awkwardly so.