Well, I think you just hit the nail on the head when you said, "...she's afraid she'll be made fun of because of the age seperation."
I think you are right about that, and I think that, unfortunately, she would get a LOT of hassle from the kids her own age for going out with a freshman, even a very mature one.
I'm not saying it's fair or right, I just mean that's usually the way it is, in high school.