Board Thread:Questions and Answers/@comment-7141877-20130315031635/@comment-4848837-20130318230330

According to an original series episode, Ben's birthday is in December. That same episode also says Gwen shares the same birthdate.

Derrick has confirmed the December birthday, but hasn't affirmed a day, but he says the goal is for Ben's birthday to be on December 27, 2005, the premiere date for the first episode of Ben 10 ever.