User blog comment:Rassilon of Old/Young Justice spoilers at Comic-Con 2012/@comment-4967138-20120918233031/@comment-4928331-20120919132318

His Birthday is on December 1.

Proof:

http://www.worldsfinestonline.com/wp-content/uploads/2012/09/yj20fp1.jpg

http://www.worldsfinestonline.com/wp-content/uploads/2012/09/yj20fp2.jpg