I'd say it's a personal choice, and it also may be something you want to observe, as in, see how old most trick-or-treaters are in your neighborhood. I trick or treated until I was 18 (the neighbor at that point said I looked "too old" to be trick-or-treating, sniff), then after I hit 18, I just would walk around neighborhoods or malls to show off my costume, or I'd open the door wearing my costume and wear it to work, of course! In my parents' neighborhood, it was common to see high schoolers trick-or-treating. Now, I can go out trick-or-treating again...with my kid, so take THAT, mean ex-neighbor, hah!
I still dress up to work, I am the only one in an office of about 40, and I couldn't care less. It's funny because one day everyone in the office pretty much dressed up years ago, and people still recall that day with fond memories, as if it's something out of a legend, yet they won't dress up anymore because they say morale is down and "it's not the same anymore". I think participating in such festivities brings morale up! My desk area is decorated to the nines and I have candy. The cleaning crew says I am fun, vivacious and have spirit and to please not take down my stuff, they look forward to laughing every day at my decor, grabbing some candy, and showing their other co-workers my elaborate decor :)
I take my daughter to school in costume and parents stop us to take our picture of us dressed up and looking pretty, and her classmates all stared in awe one year when I dressed up as Belle, they said I looked just like her. Halloween is my favorite holiday, a chance to be whoever you want to be and have fun without being judged.