Kendra Wilkinson Wishes Hugh Hefner a Happy 91st Birthday: 'I'll Celebrate You Forever!'

Hugh Hefner turned 91 on Sunday, and his former girlfriend, Kendra Wilkinson-Baskett, sent him a sweet birthday message via Instagram.

The throwback pic shows the Playboy founder cozying up to Wilkinson, whom he dated from 2004 until 2008, and a bottle of champagne clutched between them.

WATCH: Kendra Wilkinson Shares Sweet Photos With Son Hank at Bingo Game Night

"Happy birthday Hef!!" the Girls Next Door alum captioned the snap. "Thank you for EVERYTHING. People might not understand you but that's because they've never had the chance to know you like I do. I'll celebrate you forever!!!"

Also chiming in with birthday wishes was Hef's 25-year-old son, Cooper, who shared a shot of his father wearing his trademark silk smoking jacket and captain's hat.

"To the guy who is smart enough to create a job that allows him to roll out of bed in pjs and go straight to work," Cooper wrote. "Happy birthday Dad. I love you."

MORE: Hugh Hefner Returns to Twitter With Father-Son Photo Celebrating Thanksgiving

The magazine mogul himself has been quiet on social media lately, but a new docu-series explores the wildest days at the Playboy mansion. Check out some of the action in this video.