Actually, it's never been officailly released. Remember, those sites can edited 由 anyone, and rumours of his age can spread fast. So it's not been released.
posted 一年多以前
I just checked that site and it says he's turning 16 下一个 year. That would be impossible, since he's currently 18, 或者 so, after the one 年 time skip.
The characters' birthdays dates aren't mentioned in either the 日本漫画 或者 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.