Hi, Patty! This can be quite a dilemma, and I have faced it myself several times, as I often travel by myself with my girls, and the older one was always able to do more than her younger sisters, but often times didn't want to do it by herself. It makes it very hard for someone not to be disappointed.
Some of the rides have single rider lines, and this allows your 13 year old to go through what is usually a shorter line, and they will put her on the ride where they have an empty seat next to another guest. This allows her to ride next to someone, so she isn't alone in a seat on the ride.
The only other alternative is to try and pair up with another family with children the same age. This sounds like a tough thing to do, but prior to your vacation, check out places like
www.disboards.com where you can "meet" people who may be traveling at the same time, and pair up when you get down to Walt Disney World.
For my family, I often ended up having to make some compromises with my oldest daughter. If there was something she didn't want to ride alone, like Expedition Everest or Rock 'n' Roller Coaster, she would have to skip it until we could come back with someone to ride with her. But I would give her something in return, like the opportunity to choose some dining experiences or a visit to the spa to get a manicure, or even a special souvenir for being such a great sister.
I hope you find something that works for your family! Have a wonderful trip!