I can't see blocking adults from wearing costumes on board ship. Enforcing the rules already in place about weapons and masks, sure, but it would be a totally different experience if no adults could dress up. My pirate costume was a shirt, pants, and vest that was so comfortable to wear I wore it again the last night of the cruise just for fun. Pirate is cruise casual, right? As long as everyone is reasonable, I can't see costume rules changing, seeing so many people dressed for a theme night is really fun.