I went to school in their suburban neighborhood during their time frame. The cutoff during the years they would have started school was the calendar year - December 31. (I was the youngest in my grade due to my end-of-year birthday)
There’s no consistent date in every school district in the U.S., but September 1 is the most common.
Yes, I know - not sure why you felt the need to point that out. Writer/filmmaker John Hughes is from the school districts and may well have assumed an end-of-year cutoff. My point is that Sept being common now is completely irrelevant.
Ok, it is possible for them to be the same age but for only for 1-3 month window. Ferris’s mom getting pregnant immediately after childbirth just for the sake of the math working out is unlikely, so we’ll just figure 2-3 months.
Research says the titular day off is June 5, 1986. For them both to be 18 it must be within a 2-3 month window of their birthdays. Ferris turns 18, 2-3 months before Jeanie turns 19. So if Ferris and Jeanie are 18 in early June, her birthday has to be June 6-30(Ferris May or April) , July (Ferris June 1-4 or May) or August (Ferris May or June 1-4). There’s a bit of wiggle room in the months, but not much.
That changes the kindergarten enrollment dates. Neither are near the deadlines. Ferris would be April, May or June 1968 to be 18 in June 1986. The only thing I can conclude is that Jeanie was born in June, July or August 1967 and was held back a year. That might add to her resentment, being in the same class as him despite being a year older (almost).
I don’t think they are twins. They would have mentioned that, plus the tropes are that twins have more of a bond. Older brother protects younger sister. Older sister resents younger brother.
Except, they would only both be 18 between those dates: Jeanie would turn 18 at the beginning of the school year while Ferris would be 17 the whole school year, which almost everywhere ends in June at the latest. Ferris would have to be less than 9 months younger than Jeanie, since the story takes place while school is in session, and Jeanie has apparently not yet graduated (which often happens in May). Unless Ferris was premature (and he is, after all, a bit runty), it is hard to make the math work.
Maybe she is his half-sister?
Curiously, Jennifer Grey is 5 days short of exactly 2 years older than Matthew Broderick, but neither of them are particularly close to being an Abe Vigoda.
Moderating: This discussion ended up really hijacking and dominating the origin thread. So I moved it to its own.
I was just thinking the same thing. Thanks.