No, I'm pretty sure that an official age requirement to hang up your fake fangs has not been set. You'll look pretty odd attending a costume party in a tux, unless of course that would be a costume for you. Adults have costume parties all the time, & you'll find charity costume parties, college come as your favorite nude Roman & Greek costume parties. So have fun
Dressing up is just a fun thing for all ages. 18 is too old to trick-or-treat, but never too old to dress up. I am 37 and I dress up and it make thes kids in my area and family happy. They seem to love it! Last year, I dressed as a female pirate and looked like I should have been in POTC. I made the costume, which added more pride, because the ones online looked too fake or trashy. This year, I may dress as Xiao Qiao from the Dynasty Warriors games (I found a cosplay website that sells these). My nieces and nephews also love when I dress up on Halloween.