Depending on where you are, people may or may not frown upon grown adults trick or treating. Like in my area, for example (I'm 19). I would say go ahead and try though if you want to do it that badly. The worst that can happen is you might get some rude stares or a disapproving nod.
Personally, I've never been one for the Halloween holiday. I enjoy seeing people's costumes, but I'd rather sit at my heated home watching TV whilst occasionally grabbing some of the candy reserved for the trick-or-treaters. The last time I dressed up was my freshman year in high school, where I was a knight and I got some guy with two coconuts to follow me during the school parade. I won first place for "funniest costume".