I think the decision should be up to the kids as long as they are respectful and make an effort to have a costume. I wouldn't want my kid to be left out of a group of friends going trick or treating if he or she wanted to go.
The last year I went was around 12 or 13. It was my decision to stop going door to door. Since then I participated by watching scary movies and handing out candy from home. I still dress up
I was a gladiator this year.