A few things here...
1] Since the setting of the game is in New England, I'm going by the grade system in the U.S.
2] This is not a complete list. I know that there might be other hints to characters' ages, but these are the ones that I have dug up.
NOW THAT THAT'S OUTTA THE WAY...
Let’s start with the confirmed ages:
Jimmy is 15, Abby is 70 (conversion dialogue), Mr. Galloway is 34 (cutscene dialogue), and unused audio from Mr. Hattrick tells us that he is 48.
Now let’s dig a little deeper…
- Mr. Sullivan exclaims that he just wants to “take forty-two years of frustration out on that bastard!” Now, that doesn’t mean he is 42; he could be older than that.
- Mr. Wiggins claims that he’s been smoking for 50 years. Unless he started really young, this statement would make him 60+ years old.
- All the prefects are seniors (17/18 years old). Now, maybe some have failed a grade making them older, maybe not.
- Lefty, Lucky and Vance might be 17 or 18 years old, as they are seen working on cars in shop class, which is senior work only. Of course, they could just be in an advanced class and therefore younger than that age.
- Pinky claims her father will give her an Italian convertible when she turns 16. This obviously tells us she’s younger than that.
- Bif is unsure if his trust fund takes effect at age 18 or 21. I'm pretty sure the answer is 18 and if he doesn’t know that/gotten the money yet, this statement makes him younger than that.
- Chad states he’ll be 18 in a couple years, making him 15 or 16, depending on when his birthday is.
- Derby could be 18, as he wonders why his father won’t let him cash in his trust fund. “Could be” is the keyword here; he might be younger and thus denied of accessing the money at the moment.
- Stating the obvious: Dan is Thad’s older brother, but by an unknown amount of years.
- Kirby brags about being the youngest student at Bullworth to receive a varsity letter. I don’t know what the youngest age would be to get one (or if age even matters), but his voice tips me off that puberty is hitting him during the game. Maybe he’s 14 or so?
- Trent was held back a year (maybe more?) so he could be older than other students in his class. Not that we’re sure if he’s a freshman/sophomore/what have you.
^ As a side note, does that even matter? I mean, Beatrice, Bryce, and Sheldon are seen walking into Jimmy’s classes. I don’t understand the latter, considering he’s a little kid.
^ And on the topic of little kids…
Bif: “I wonder how many seven year olds I could beat up?”
Hal: “I could use some cash. I need to hit up an ATM. And by that, I mean an eighth grader.”
Eighth graders are usually 13 - 14 years old, and these little kids don’t look like they’re even close to hitting puberty. While Bif might not be referring to the little kids at the school, there is a possibility that they might be around 7.
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Thoughts? Opinions? Go nuts.