Sometimes it’s surprising. Actors playing roles that are 10-12 years younger are common. And plenty of examples of 15 years younger.
Sometimes such roles are just stupid. See Babs Streisand in Yentl.
Or Howard Stern breaking the Third Wall and saying “I know what you’re thinking. That guy’s too old to be a college student. Well this is MY movie, so you’ll just have to deal with it!”