I really don't mind the teens going trick or treating so long as they behave like the rest of the trick or treaters and don't push in front of the little kids etc. I've been really impressed with the teens that have come to my door and the respect they have shown. I would much rather have them out trick or treating to get candy than have them out doing some sort of vandalism, egging kids or cars, causing damage, drinking or whatever. The only thing I draw the line at is that I stop giving out about 9:00, I think that's late enough and I'm usually out of candy by then.
Kids can be cool at any age and fun and harmless, and should be commended for not causing trouble I think. My kids are older now but they did go trick or treating till they got too tall to pass for a young kid! And now they stay home and help me make special treat bags for kids we know and take around a couple of little kids.