How old is Ferris Bueller's sister?
Let's look at what we know:
1) She's Ferris' younger sister.
2) Ferris is a senior.
3) Seniors are usually 18 or 19.
4) She has a car.
5) The legal driving age in Illinois is 16.
6) When arrested, she's treated as a juvenile.
SO: Ferris' sister is at least 16, she can't drive if she's not 16. She can't be more than 18 and be Ferris' younger sister. When arrested she's treated like a juvenile, so she can't be 18.
The answer above is incorrect. If you read the script which is posted online [http://www.idiotsavant.com/bueller/script.htm], Jeanie is described as the older sister.