If you are accompanying a child no limit, but older than 10-12 can be a little creepy, but then again the whole idea of Halloween is to be a little creepy. What a conundrum.
My mom once had an acquaintance who was 42 years old and still went out trick-or-treating on Halloween. She had a costume, too. One year she was a cowgirl.
We get trick-or-treaters up to age 16 or so. Happy to watch them laughing and having fun with their friends. It's not easy to be a teenager and an occasional revisiting of their younger years isn't out of the ordinary.
I pass out candy to all who trick or treat at my house, adults included.