Poll: How Old is Too Old to Trick-or-Treat?

Much like baseball, there are unwritten rules that come with Halloween. 

1. Don't take more than one piece of candy from a bowl.
2. Don't leave carved pumpkins on your porch until they rot.
3. If you scare a kid and they cry, you better makes amends with A LOT of candy.

There is one part of Halloween that still remains unclear to me. How old is too old for someone to trick-or-treat? 

Obviously there are variables that come into play: college town vs suburb, parents accompanying kids, etc., but let's get this settled once and for all.

If there was an unwritten rule in Dublin, what should the cutoff be?

Please explain your reasoning in the comments section.