I think the last time I went real trick or treating I was 8. After that we moved to New Mexico and we'd go to the alternative parties instead.
I think as long as the kid is dressed up in some kind of a costume, the only age that is too old is when it stops being fun.
My dad was telling me about how one Halloween he and his younger brother were out driving around and saw kids trick or treating. They wanted some candy, so they stopped and Jim (dad's brother) got out of the car and rang a doorbell. No costume or anything - maybe college age? - he got some candy.
(Jim and his wife are now missionaries in Cyprus.)