Unfazed by a mere 60-year age difference, Playboy founder Hugh Hefner, 84, proposed to Playmate and

Mansion roommate Crystal Harris, 24, announcing their engagement via the even younger Twitter. "This

is the happiest Christmas weekend in memory," he tweeted. It will be the third marriage for Hefner, who

celebrated his first nuptials in 1949. Harris, Playboy's Miss December 2009, recently appeared as Hef's

"No. 1" girlfriend on E!'s show The Girls Next Door.