Actually, it's never been officailly released. Remember, those sites can edited by anyone, and rumours of his age can spread fast. So it's not been released.
posted over a year ago
I just checked that site and it says he's turning 16 next year. That would be impossible, since he's currently 18, or so, after the one year time skip.
The characters' birthdays dates aren't mentioned in either the manga or the anime. It might be in some other stuff though - I don't know. I think there's a fanbook - it might be in there.