The PP&P,
MNSSHP and
MVMCP are the exceptions to the adult rule. Adults are encouraged and allowed to dress up for these events.
And actually your comment about a couple of them being good enough to attract crowds is the reason for the rule the rest of the days.
For the person upset about the 10 year old......I would imagine that they officially cut it off there because that is the age that your admission ticket is considered "adult". No way would I consider my 11 year old an adult, but I have to pay for her as an adult both admission and food. But as I said in my other post, she dressed up as Belle (winter dress) and paraded around MK all day long and although lots of people took her picture, and one very young girl, maybe 4-5 asked if she was a real princess, there was no way that someone could actually think she was Belle. I can imagine, however, another couple of years and that not being the case,

...she's actually talking now about wanting to work at Disney maybe as Belle (gonna be hard since she'd blonde).
If you have a 10 - 12 year old, or even a young looking teenager, I'd would suggest that you talk to them about the official rule. Let them know that they are welcome to wear their costume but that if a CM requires it, they'll have to pretend it's midnight and change back into their real clothes. They're old enough to understand, and even if disappointed, hopefully won't have a meltdown because of it. Rent a locker to put regular clothes in and switch if required. I think the tragedy here could be not informing the pre-teen beforehand and having them refused.....that could lead to some unhappy times.
And for those that feel it necessary to flame about "a rule is a rule"...go ahead....I have my flame proof undies on.