John Stamos Shares Sexy Underwear Pic on 52nd Birthday, Gets Surprise on 'Grandfathered' Set!

ET surprises John Stamos for his birthday on the 'Grandfathered' set.

John Stamos turned 52 on Wednesday and he rang in the day with an Instagram photo that proves he only gets better with age.

"At 52, proof I put on my undies like everyone else ... one leg at a time," he wrote as the caption to a photo showing the shirtless actor slipping into a pair of slacks.

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ET helped celebrate Stamos' birthday by surprising him with baklava on the set of his new Fox sitcom, Grandfathered. Guest star Richie Sambora pitched in during the surprise celebration, singing "Livin' on a Prayer."

On the upcoming comedy, Stamos plays a playboy restauranteur who meets a twentysomething son he never knew he had. Making matters more complicated, his son has a daughter, making Stamos' character an instant grandfather.

The show uses twin toddlers to share the role of Stamos' granddaughter on the show, not unlike how the Olsen twins Mary-Kate and Ashley played Michelle Tanner on Full House.

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"When these girls start getting fussy, I say, 'I made some other twins very wealthy. Be good and you can have your own clothing line,'" Stamos told ET.

Watch the video for more. Grandfathered debuts Sept. 29 on Fox.