Any age is fine with costume! I don't turn anyone down and I'm happy seeing big groups of kids or teens or college students dressed to the 9s enjoying the holiday. There were a few years in college (when I had time) that I made 150 treat bags for the first people who showed up regardless of whether they were kids or not. When I was 12 years old I was trick-or-treating alone as a grim reaper; I said "trick or treat!" to a man who opened his door and he said "aren't you too old for this?" Then he stared at me, went back inside and found some old cracker pack to give me, and turned off his light for the night. That man ruined Halloween for me until I was 17 years old and rediscovered the holiday by watching Halloween specials on TV. I would never want to put any other child or person through that. There is no reason to destroy the spirit of Halloween just because you think someone is "too old" to celebrate.