The overwhelming majority of people believe there should be no age limit. They have two reasons for this: 1) we're all kids at heart and 2) they would rather people be trick or treating than playing pranks and causing trouble.
Some say as long as they have a costume (or at least some facsimile of a costume), they'll hand over candy. But all too often these too-old trick or treaters are easily identified by their complete lack of a costume.
Apparently, however, if you do trick or treat as a teen, you should be prepared to be confronted by an annoyed homeowner at some point. That may seem wrong, but it's actually against local ordinances in some states to trick or treat after a certain age.
The reason for the ban is the very same thing I've been arguing for about three years with these people. You see, I lived in a neighborhood that was in rapid decline. After the 2010 flood, you could buy a house there for $80K, so some shady elements were moving in. Those shady elements had no problem letting their teens roam the streets all day. On Halloween, those teens came out without a costume, grocery bag in hand, cell phone pressed to one ear as they held the bag toward you and waited for you to fill it with candy.
Or will they? Over time, as more and more people accept uncostumed adults on Halloween night, more and more people will be doing this:
Which sucks because THIS is so much fun!
Does your city have age limits on trick or treating?