"I'm 80 and I can't believe it!" Allen told a news conference. "I'm so youthful, agile, nimble, spry, mentally alert that it's astonishing."
"Cafe Society", a romantic tale of 1930s Hollywood, will be screened on Wednesday.
It is the third time a film by Allen, who does not enter them for competition, will have opened the festival, after "Hollywood Ending" in 2002 and "Midnight in Paris" in 2011.
It may not be the last.
"It's great but I don't feel old. Now I'm sure one day I'll wake up in the morning and I'll have a stroke ... and I'll be one of those people you see in a wheelchair and you'll say: 'Remember him? He was Woody Allen.'
"Until it happens I was going to continue to make films as long as people are foolish enough to put up the money to support me."
Asked how he stays fit during long hours on set, he said: "I don't know, I eat well, I exercise. It's luck, my parents were old, my father lived slightly over 100, my mother lived almost 100 so if there's anything in heredity I hit the jackpot."