I agree...I think being older than 16 and trick-or-treating is quite a bit on the creepy side as well. The one exception is when you have kids and they want you to dress up.
Well obviously when you have kids it doesn't really matter what your age is. You'd definitely get some funny looks from people though, and I can imagine if you went with a couple friends it might be really unsettling for like an elderly person living alone to open their door to find three six-feet-tall 20-year-olds and be unable to tell who they are through their costumes.