Dial down the enthusiasm for partying
The easiest way to overcome her assumptions about your age is to separate yourself from wallin' frat boys who are doing their best impersonation of Frank the Tank.
She's been at school for a few years, and frat parties are nothing new to her, which means she's less likely to be projectile vomiting on her fellow revelers. But you're fresh on the scene, and she'll fully expect you to be an amateur booze hound who can't hold his liquor and is so high on his newfound independence he wants to party all the time.
Act apathetic in the presence of kegs, resist the urge to be on ten every night, and give the impression that you can handle your damn self, and she'll consider you above the likes of your wilding peers.