Happy Birthday Jon,
I don't remember what I did for my 18th birthday, but I'm sure it involved boozing it up. I don't think I did anything spectacularly different. Honestly, 18 didn't seem like that huge of a deal for me. I was a freshman in college and already partying relatively regularly. I do remember my 21st birthday (had a party) but the rest are lost in the fog of time....
But maybe what you should learn from all of this is: Start writing down what you do on every birthday. Most of mine are un-recallable at this point, but it would be kind of fun to be reminded of whatever it was that I decided to do each year. I'd love it if I had a little "annual diary" of whatever I did and what seemed important to me at that point in my life...
Have a good one.