My girls still enjoy dressing up for Halloween (and going out trick or treating). I figure, let teenagers enjoy it. They could be doing much worse things.
I dress up with them (it's fun to pass out candy when you are dressed up too. Or if I go around with my girls, I feel like "one of them".)
I have been an indian, Minnie Mouse, and this year I am going to be Rosie the Riveter. I put together costumes, for me and the girls, based on items we already have here at home. We may have to purchase a couple items, but for the most part, we use what we have and items from our dress up bin. It's fun to dress up! I think you should do it!