(Burbank, CA) -- A Houston-born actor is walking away from a 50-million-dollar deal. Jim Parsons has turned down the offer to appear in a 13th season of the popular CBS TV series, "The Big Bang Theory." Parsons, who plays the eccentric Dr. Sheldon Cooper on the show, says he believes they've done as much as they can with the concept. He also points out he's now 45, a bit old to be playing a character who always wears T-shirts. The character is central to the show, so this season will be its last. The final episode will air in May.