I'm gonna be a little math nerd here and go on a number-based rant, I apologize in advance.
Ollie openly states he's 26. The protagonist answers that that's only 1 year older than him (so protagonist age is 25)
Rofi went to school with the protagonist, so the assumption of them being the same age is easy to make.
Artemis makes a point to say Theo is 30-something, and at some point I believe the protagonist even says Theo is around 10 years older (I remember reading that, but I can't be bothered to re-find it to check whether it's around 10 or exactly 10), so let's say Theo is 35.
While Artemis and Ollie met while Ollie was a junior, and Artemis only spent 1 year at college, we can't actually be certain of his age as Artemis is the only character that strikes me as "would take a gap year before going to college" kind of person. Maybe more than a year? All I know is that everyone except Theo is younger than 30.
Hunter got tutoring from Rofi during Rofi's final year at college, but I actually don't know how to use that for calculating purposes. Sounds to me like Rofi was already finishing college while Hunter was just starting, but I don't know how long did Rofi's biology major take. Hunter also doesn't strike me as a gap year kind of person, but I don't know much.
Using the in-game given info of Theo turning 17 in 2005, the earliest year the numbers make sense would be 2023 as the year the game takes place in.