I have to chuckle because I've often thought Disney cuts off the age for a child at 9 (instead of the more appropriate 11 or 12) because half the so-called 9-years-olds really are 10 or 11 or even 12! If the age cut-off were 11, people would be passing their 13-year-olds off as 11. Seems to me, Disney has already figured out most people knock a couple of years off a kid's age and has dealt with it by making the cut-off artificially young!
As for taking advantage of Disney, I don't personally condone what the OP's sister wants to do, but I simply can't accuse anyone of taking advantage of Disney when their top executives often take home, with stock option exercises, something like $500 million annually!
Finally, is it hard to tell someone's age. You bet it is. My dh was recently carded at the state liquor store -- and he's 48!!