This game is written by a guy isn't it. No way an 18 year old girl today's date is so naive, unless they're "special"
Emma is clearly not meant to be 18. I know that the game say she is in a college, but the whole vibe of the "College" is more like a High School. I see and play Emma as a 15-17 year old kid still comming to terms with her sexuality with some serious confidence issues. All the other kids in the School behave like HS kids and not like college at all.
(Compare that with Suzie, where it has a proper College/Univercity feel.)
I might be mistaken, but I'm not sure that her actual is mentioned anywhere. Her age is not a primary focus on the game, but rather her confidence, naivity and innocent and still a tender age. (A girl not yet a woman)