I Now Assume that Everyone I Don't Know is a Freshman

I've noticed while walking around campus now that I just sort of assume that everyone's a freshman. I have no evidence to support this assertion seeing as how my school is like 7,000 people and a significant majority of those people must in fact NOT be freshman... but I still can't quite shake that feeling. Given how there's very little difference in the looks of an 18 year old and a 22 year old I have probobly already assumed that multiple 5th year seniors are freshman. And this goes without mentioning that one girl I've seen walking around that I'm pretty sure is roughly 12 years old.