I fly F out of SEA a lot and haven't had this happen despite being not only (relatively) young and female, but also fairly overweight (which for some reason some people tend to assume = less money/lower class) and have always assumed it's because everyone just figures you're probably some dot-com tycoon.

I have had it happen in line at check-in elsewhere, though more often of the "you know this line is for ELITE passengers, right?" variety to which I sweetly say "and First Class." The best, however, was on a BA flight from LHR to SEA, when I was flying in F (on an award ticket, I hasten to add) and my mom was flying in coach (for the record, I offered to upgrade her to biz and she turned me down). Midway through the flight, I desperately needed a painkiller which I knew she had, so I wandered back to the cheap seats and talked with her a bit and got the pill, etc. As I was making my way back to the front, a FA stopped me and told me to go back to my seat. I said, "yes, that's what I'm doing. My seat is that way" and pointed to the front.