Haven't for over twenty years - and I was still pushing it at fourteen. I decided while wearing an Elvira costume and getting wolf-whistled at that I looked too mature to be trick-or-treating. I don't miss it. As an adult, I can buy the candy I actually like (instead of getting a million Tootsie Rolls and Dum-Dum pops).
I'm disappointed that no one bothers to dress up now - the kids don't even bother to SAY "Trick-or-Treat!" either. It was sooooo much better when I was a kid.