A friend of mine just turned 40. I was going to buy him a "I'm a Man, I'm 40" mug or t-shirt. However, I didn't buy it because I eventually realized that he and all my geek friends would have absolutely no idea about the origin of the quote. It was in sharp contrast to my cousins, who all immediately understood "You're a man, you're 40", when I wrote it in a cousin's birthday card.
So as an educational service, the quote comes from Mike Gundy, Oklahoma State's head coach, in 2007. You can see it below at about 2:15.