I have had people in my dept. ask if I was in my late 20s before (I'm a CA, I associate mostly with the RAs, who are all my age or older). I'm 20.
of course, it didn't help that I generally associate with the older RA's (we have a few in their mid-20s) and the RD's.
Well, not just that, I've had residents and others around campus mistake me for being in my late 20s.