My son is 5 and he just doesn't do cartoons anymore.
He watches How Things are Made, Dirty Jobs, and Myth Busters. When not tuned into the Discovery Channel, he is usually watching whatever sports he can find: Baseball, Basketball, Hockey, Football, Golf, Tennis, Billiards. If it's competitive he's usually watching and cheering for one team to win.
My 3 year old DD is still loving Little Einsteins. SHe's been watching obsessively since around 18 months. She's limited to one 22 minute episode a day. IMO, it's plenty for her age.
