Other mistake: Jack's parents were at a Halloween party when his mother went into labor. Later in the movie when he's asked by his teacher he says his birthday is September 12th.
Jack (1996)
Ending / spoiler
Directed by: Francis Ford Coppola
Starring: Robin Williams, Diane Lane, Jennifer Lopez, Bill Cosby, Brian Kerwin
By the time Jack graduates high school, his body has aged to that of an 80 year old. He goes off to a graduation party with his friends.
Ben Wallace
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Suggested correction: The movie doesn't specifically say this is a Halloween party. This may simply be a costume party which can and do happen multiple times a year.
As you stated, it doesn't say it's a Halloween party. In fact, there's signs in the room that say "Beaux-Arts Ball."
Jack: Hey guys, I brought a friend, okay? This is Mr. Woodruff.
Eddie: Are you ten, too?
Lawrence Woodruff: Uh, well if you ask my wife. No I'm afraid I'm just a regular, garden-variety old fart.