Disney World is very disability friendly. If you have a disability, including a non apparent one, go to Guest Services at the first park you go to and request a disability pass. They will not require any proof. They'll ask your name, how many people are in your party, take a photograph of you with an ipad, and have you sign the pass.
Go to the standby or the fast pass line (choose whichever isn't clogged at the time) at the start of each ride, and hand them your pass. They'll fill out a return time based on the current line wait, and stamp it. You can then return to ride walking through the fast pass line at the return time. Unlike fast pass, the time doesn't expire in an hour, you can use it the rest of the day, but you cannot get another ride scheduled until you have the first ride crossed off. The person with the pass must be a rider, so the rest of the party cannot use this pass without the person that is was meant for.
Just to be clear, you can only have one ride scheduled at a time. Once you've completed that ride, you are free to get another.
Disneyland is similar, but seems to have more discrepancies, and more CMs willing to be nosy about disabilities.