Actually, it's never been officailly released. Remember, those sites can edited Von anyone, and rumours of his age can spread fast. So it's not been released.
posted Vor mehr als einem Jahr
I just checked that site and it says he's turning 16 Weiter year. That would be impossible, since he's currently 18, oder so, after the one Jahr time skip.
The characters' birthdays dates aren't mentioned in either the Manga oder 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.