Never. It's a time to have fun, age shouldn't be involved! I'm 14 (in high school) and I go trick or treating with my friends who are the same age! Even tho we seem like retards going down the street, racing each other to each and every house in the survey! Then counting who got the most! I'm never stopping! Heellloooo free candy and FUUUNNN!!!! XD
When you start wearing makeup or caring about what you are wearing, you probably are too old. I always told my kids that if they want to do Halloween after age 13, to help hand out the candy and/or walk with their siblings or younger kids as they trick or treat.
whenever it starts getting lame. I'm 13 and still go anyway with friends because we think its fun and its one of the only times we can pretend to be fictional characters without looking like freaks. ;D