The source is When I'm Sixty-Four, written by Paul McCartney and released by the Beatles in 1967 on Sgt. Pepper's Lonely Hearts Club Band. The original is a playful promise about growing old together, sung in a light music hall style. Online editors altered the age to sixty nine, turning that wholesome premise into a brief innuendo gag. The altered line spread on soundboards and in short meme edits, where its familiarity does most of the work and the changed number supplies the entire joke.