I've complained about a lot in the past, but this is just about the dumbest thing I've ever heard.
There is no reason to be afraid of kids or even people my cousin's age trick or treating. I'm curious, what is the fear based on anyway? You think they're going to immediately swarm into your house and cause mischief or what? Why worry about somebody asking for something that you have plenty of and are willing to provide for free? That's what I see it as: asking for free food. It may be mostly junk food, but it's still food. That's all these people, both young and old, are really doing is looking for something to eat.
Or is this all just based on social stigma? Deviation from social norms is not reason enough to make a law limiting people's freedoms. The fact that it's not a normal occurrence does not make a cause for a concern. It doesn't mean that there is something wrong with a person if they deviate from social norms. Then again, considering the history of the human race, this isn't a surprising thing to discover, and just proves that even the smallest little thing can be taken far out of context.
No matter what, though, there is no good reason to ban anyone from trick or treating on Halloween. Anyone deliberately out to get someone in any way on Halloween is not going to be trick or treating. They obviously have other motives if they're not, and that's all you'll be giving people (particularly teens, I imagine), a reason to do something else, including trashing houses, like has always been done and can be done even if not allowed to trick or treat. So even if it's banned, there's no point, because shit can still happen anyway. The only purpose it serves is to limit citizen's freedoms for no reason.
So why do it?