In 1977, to Dick Cavett while accompanied by Sophia Loren; quoted by French Film Stars Database http://hri.shef.ac.uk/filmstars/starsDetail.php?intID=978&strRecord=Newspaper_Article, which sources it to his obituary in The Guardian
Marcello Mastroianni: Frases en inglés
“When I was young, life seemed long and endless to me.”
As quoted in "Latin Lover"? No? How About Lover Of Life?" in The New York Times (13 August 1999), p. 116