I agree with the "If you like it, continue to do it" comments. That's how I feel. I stopped trick-or-treating when I was about 16/17 and not necessarily because I wanted to stop, but because my mom wouldn't help me out. I'm 22 and I don't trick-or-treat, but I would still dress up if any kids came to my house. I know a lot of people trick-or-treated until they graduated high school.
As garmanma said, if you say "trick-or-treat" and "thank you", most people who hand out candy won't mind how old you are. I think there will always be those people who think that if you are over the age of 8, they will think you are too old to do it, but as far as I'm concerned I think it's a holiday for those who want to enjoy themselves and have fun, and of course want a little candy. So, you keep doing it as long as you have fun doing it.
"I would feel more optimistic about a bright future for man if he spent less time proving
that he can outwit Nature and more time tasting her sweetness and respecting her seniority."
E. B. White (1899 - 1985)