Before I write this i should say this ONLY pertains to mnshhp, I don't see a reason for adults to get dressed up for everyday, and that has always been in rules that they could not dress up on regular days. I'm also talking about mnshhp on Halloween itself mainly.
In DHS on jan 1st there were 2 adults I saw in costume, so obviously at moment its still as discretion. They were in full body animal pj costumes. I spoke to 2 cms about it and got different stories. One stated that it was ONLY costumes that could explicity be assumed to be a park costume (Princesses, haunted mansion guests, etc.) another told me it was all and that I shouldn't even buy a ticket for mnshhp if I liked costumes. So obviously no one really knows yet.
I personally cannot see a company that makes money off a halloween party for all forbidding for 14 plus over on halloween itself and possibly not for the other parties. A majority of families taht go will have children that are under and over 14, if they know going will only allow half of their children to participate they may refuse due to it being unfair to their kids. They also cater to a lot of people with disabilities, a parent with a child with an intellectual disability who is over 14 in age yet not in other ways may not take their child if they cannot enjoy it the way they want and they already had a lawsuit with ADA stuff. Also, when I went on Halloween it was more adults then children dressed up, we know these rules because of being on here, will they really turn away a bunch of people who show up who had no forehand knowledge? To me for a company that in the end is wanting to make money wouldn't want the bad publicity involved in turning any amount of people away.