"Disney Years" is adding the two numbers in your age together So, my mom was (is) 13 instead of 58 because 5+8 = 13
Among other things, some of the teenage girls costumes were just a bit too short, it's those sexy costumes that are pretty popular - if half of your butt is showing, tights or not - it's a no in my opinion.
Some people dressed up, some didn't - some just wore a halloween shirt and halloween ears. I saw a lot of little boy Pirates - probably got it done at the pirates league earlier that day. You can always do Mike and Sully - get a t-shirt of each of them, black pants and see what hats/ears disney store has online.
You can always "Disney Bound" as a costume, I saw a lot of girls doing that, but you can do and be anything! If you are brave enough, you can wear your pj's and messy curlers in your hair and look like you just got out of bed! I was desperate one year and I did that at school, lol.