Obi Wan Kenobi said:With the greatest of respects, It's still theft, the CM from his employer and your sister from Disney.
When you book flights fro example, the child turns of age during the trip, you pay for the return at the new price.
From what I have always understood is Disney have a policy of not trying to give you a hard time on your trip. I bet some place it says if a child starts a the holiday 1 age and end it another age just go with what the start of trip age is. I bet they done a cost study that worked out it cost them more to deal with someone changing age during a trip and figured that by time everyone is trained to deal with it, computers upgraded and time and inconvenience caused to customer and staff that it would not be cost effective.
I do see you point but I would imagine CM's have come across this before and the reply they give is what they have been told and not what they felt like saying and to be honest the Disney World site says "Please specify child ages at time of travel" so you could argue the point that when you travelled to Disney your child was 9 because that is indeed true.