I got into an argument with a friend of mine who said it is okay for adults to do this. I always felt that it was okay for kids who are 14 and younger. Once you turn 15 I think it is best to stop. It is more of an event for younger kids anyways.
I think you are right. I would say around the same, age 14/15 should be the cut-off. I know someone who actually believes it should stop at the age of 12 which might be a fair argument in certain parts of the world where 12-year-olds are more "mature" than they should be.
I have no issue with it if that is what the person is into. Who am I to tell a person they are too old to dress up? I would much rather adults went out and did this than got drunk and stuff.
I don't think many do but those that do are often with their children. I never really thought about it. I don't see anything wrong with it unless they are drinking or causing issues for the kids who are participating.