I seem to be one of the few with the unpopular notion that trick or treating is for children - so let me explain my position. My 11 year old has no interest in trick or treating this year and didn't last year either. I never told him he had to stop. My 9 year old will probably trick or treat for longer - however I won't be taking him past elementary. If he wants more Halloween fun, we'll have a party. Trick or treating just isn't that big of a deal here. Teenagers and adults tend to go to parties or help little ones celebrate at carnivals and such. I've never had an adult come to my door and hold out a bag for candy - around here it is only children.
Every year we go to two Halloween carnivals, one at school and one at church. This year my 11 year old is helping right beside me. We are doing cotton candy at school and running the cupcake walk at church. It was his choice to be a helper rather than participate and he is looking forward to it. I think it is a natural progression and would find it very strange if he continued to just want to participate. Past elementary I would strongly encourage him to be a helper and simply wouldn't take him to be a participant. When I was a teen it was the same - we helped at the carnivals and some years went "trick or treating" for unicef or canned food.
For us, it is the same with all the holidays. This is the first year at my house that "Santa" isn't the only one filling the stockings. My kids are helping me be Santa for their father. Last year was my oldest's last Easter Egg hunt. At our hunt (several families) the rule is that anyone older than 10 hides the eggs and the younger kids hunt them. From an early age, being involved in the holidays has meant helping to make the magic for others - not just receiving. I had some friends who have gone through the whole "holiday blues" thing when they realized that they had to make some of their own magic - the transition was much easier for me and I hope it will be the same for my kids.