Hi Margaret,
In previous years, Guests of all ages were permitted (and even encouraged) to attend
Mickey's Not-So-Scary Halloween Party in costumes.
There have however been some recent changes to the Walt Disney World Resort
Park Rules and as a result of this, costumes may not be worn by Guests ages 14 or older.
From what I understand, as of right now, this rule will also apply during Special Events such as Mickey's Not-So-Scary Halloween Party. Personally, I keep hoping that this rule will be relaxed somewhat for Special Events in order to allow adults to join in the fun and get dressed up too.
I would highly recommend that you keep an eye on the
Disney Parks Blog for more information on this topic, especially once the tickets go on sale (which could be anytime now).
Wishing you a magical trip filled with memories which will last a lifetime!
Emma