Mine came from 8th grade. Our family had had a computer for a month or so, but I still had no idea what to do with it, besides playing Mech Warrior that came with it. One of my buddies came over to help me setup an email account and AIM. I was completely into airplanes at that point (I guess little has changed), and he said try pilotman for a username. All taken. We tried every version we could think of, then he said, here we can type it like this p1l07m4n.
I was sitting there looking at it like, "WTF?" He tried to explain to me how it worked. I said sure, whatever. It has stuck since then. The worst part is giving someone my email address because it always goes like this, "Im just going to give it to you phonetically. It will make things more difficult on you if I don't."