I've had this happened to me many times, not only by GAs, but also by FAs, check-in agents, and lounge agents. When I was in my early 20's I always replied with a DYKWIA attitude, but I learned to stay quiet, smile (with a very sarcastic smile), and let them deal with their embarrasement.

There's always the "dress-up" argument. Even when I was in college, I always dressed business casual, and trust me, it didn't make any difference.