I'm in my 30's now, I own a house and take pride in scaring the pants off kids when they come to my house for candy. Over the last few years I have seen older and older kids going trick or treating. Why? FREE CANDY!
Put on a silly costume, run around your neighborhood with a pillow case (or two) and get free candy from all your neighbors. Heck, I know people who hit multiple neighborhoods to get the maximum candy fix.
I think if you are in the spirit and dress up, and just want to have good fun, why not? It's all in good fun for the most part. When you get the older kids who have fake (and sometimes not fake) weapons who show up at your door, I can sense why some might think older kids need to back off though...