Well, the MC is about to turn 19, so I'd reckon that Derek, Maya, and Josy are all probably 18 and soon to be 19.
Jill is in her 3rd year making her 2 years older than the MC, so she'd be about 20/21.
Quinn is either 2nd or 3rd year making her 19, 20, or 21.
Sage seems to be in her 4th year which would make her 21/22.
And I believe Bella is around 35.
As for Quinn being a LI, probably not, at least not one you can actively pursue. She's considered a side girl and, unlike the main girls (Bella, Jill, Josy, Maya, and Sage), she doesn't have RP that can be accumulated.
Not doing the threesome with Quinn and Riona has a small impact on a scene in ep 4 that I won't spoil since you're not there yet, but beyond that we don't know if it has any larger consequence.