At What Age Should Kids Stop Trick-or-Treating?

I'm torn.  Everyone who's given out candy on Halloween hates the high school kids who slap together mediocre costumes, and run around demanding your best candy.   On the other hand, I've seen some older kids dress and take their little brothers and sisters trick-or-treating.  I can't help but want to give them some extra candy for playing along.

So what age do you think kids should stop trick-or-treating?