I feel like there's no age where dressing up and knocking on doors becomes inappropriate. It's fun, it can increase social cohesion in a community, there's no reason for adults not to be a little silly, yadda yadda. Already now it's perfectly acceptable for an adult attending trick-or-treating children to dress up as well, but I think adults alone or in adult groups should be allowed to dress up as well.

But then there's the "asking for candy" part... Now I don't think there's any age where people should stop eating candy, either — but when you have the ability to easily buy (or even make) your own candy, then maybe it'd be a better idea to start giving out your candy to the houses you knock on, if you still want to go out in costume.

I dunno, just a thought. I wouldn't tell your sister to stop, though. We're all a little weird at the end of the day.