Right age to stop trick-or-treating?

I'm curious about what you think is the right age for a child to stop going door to door asking for candy at Halloween?

I ask because my cashier at Walgreens this morning told me that she plans to go out trick-or-treating again this year. She is 19 but could pass for a younger teen in costume.

I have spoken with a number of people who don't pass out candy anymore because so many people bring kids in by the carload to their neighborhoods and many of them are teens. They just don't think it is right to spend $20 or more on candy for kids who are rather old and don't even live in their neighborhood.

Thoughts?