"(Ah, the Apple Trees) When the World Was Young" is a popular song composed by Philippe-Gérard, with lyrics by Angèle Vannier. The English lyrics were written by Johnny Mercer. The original French title was "Le Chevalier de Paris". Apart from a reference to apples, the English lyrics have nothing to do with the French original poem.
English lyrics were originally written by Carl Sigman, but these were rejected by the music publisher, Mickey Goldsen. Sigman suggested Mercer, and Mercer wrote the English lyrics (three verses and three choruses) in three days.
The song is from the perspective of an aging Parisian "boulevardier"/"coquette", as they review their life.