I hereby decree…
If they come to your door on Halloween night, give them candy.
Okay, you know that. Adorable costumed children come to the door and receive candy. That’s how it works.
What if they’re not costumed? What if they’re 15 years old?
Hey, guess what! Doesn’t matter. They still get candy.
If you’re the kind of person who says to a teenager at your doorstep on Halloween “go away, you’re too old”, you just really really really need to get a life. Here’s a better idea. Teenagers come to your door trick-or-treating? Give them candy and move on with it. What exactly have you accomplished by bitching at them and sending them away? Nothing except act like an asshole, that’s what.
And the towns where it’s actually illegal for these teens to trick-or-treat? How much of a deranged sociopath do you have to be to think a trick-or-treating middle schooler should be arrested? 
So it’s the same old thing. Teens are too old to take part in children’s activities, and if they try they might be breaking the law. Teens are too young to be at (surely alcohol-oriented) adult Halloween parties, and if they try they or someone might be breaking a law. So as usual they don’t fit neatly into the hard and fast “child” or “adult” boxes, too old to be cute and “innocent”, too young to be trusted and respected, so they are cast aside for being a difficult to sort age hybrid. And where teens have their own category, perhaps their own Halloween activity, they are presumed to be vandalizing something or drinking underage or otherwise somehow breaking the law. So really, once again, teenagers can’t win and are ruining society by having the audacity to exist. 
Enough of this bullshit. This is a fun night. Someone comes to your door as part of these festivities, you pass out the candy and let everyone move on with their lives. If this is something you simply can’t do, that it’s simply impossible for you to not police the costumes and ages of trick-or-treaters, then please just turn off the porchlight and leave this holiday to those of us who aren’t complete assholes.