I always use androgynous nicknames online-I had a chatroom taking bets for a week as to what gender I was. (Helped that it was a car-based room and I'm not ignorant about cars).
The best was at a 'disco'-sitting at the bar, I started talking in a southern drawl to everyone. Finally, one guy I had been talking to said he loved my accent, what part of the south was I from? In my best southern belle, I replied, 'Oh, ah'm frum suthuhn New JAHRsey...Bo'dentown'.
I fooled many right here for two years-except most of the mods and two friends, everyone thought I was in my early 40's.

I still get that all the time in real life. It's great to see jaws hit the floor and conversation stop cold when I reveal how OLD I am.