I think there's a lot of room in the college-theme, because 90% of the games that attempt it are garbage. I think your model can pass as a college age character. The power of suggestion is real, and if you told your players that she was 21 I don't think that would break any suspension of disbelief. Now, if you told them she was 18, that would probably be too much. But these 3d models allow for the players to fill in some of the gaps, so if you tell them she's 21 I think they will accept it. It's just too difficult to really design a model that looks distinctly 21 but NOT 25.
Agreed. My own wife always looked 20 years younger than her actual age. She was mistaken on more than one occasion as my daughter! Meanwhile she was actually older than I was. She was a tiny female, only 5 foot tall (compared to my 6 foot height). On the flip side, I have always looked older than my real age. When I was 14 I had two girls who liked me lie about their age and tell me they were 18 when in fact they were 16, because they thought I was older. Heheh... the good old days.
So yeah, it's not a stretch to have a female character look younger or older than her age. I think I see breast size used to portray age a lot, with younger girls always having smaller breasts, older ones having larger. When my wife was 11 years old, she had 36C breasts (she was embarrassed and used to hide them as best she could). Meanwhile, my own mother was always very flat chested.
So you can create pretty much any character you wish and have them be any age you want (within limits of course). Reality is far stranger than fiction with far more variety in my experience.