Kaori in his early to mid teen ( 13-16) years. At some point, he cuts his hair reeeally short, then later grows it out (of course XD ). That cute goldfish he's looking at, believe it or not, is a mermaid trapped in that form, whom he frees and she eventually becomes one of his four shikigami.
Here's a pic of Kaori riding on her back in her giant koi form here: http://www.furaffinity.net/view/589246/