We used a double jogger for my twins at age 6 and probably 7 too, I can't remember the exact age we stopped taking it. Once the kids got to be these ages, we only used the jogger for certain purposes, such as when we needed to move quickly from A to B. I also loved the jogger for safety purposes when we were in congested areas - I just felt so much safer telling my girls to hop in the jogger until we got through the crowds. It was also very helpful for late at night when the kids were whipped. They'd just sit in the jogger and get a ride back to the resort...DH and I found we could take our time, grab a drink or snack, and we were all happy.
When the jogger wasn't in use by the kids, we found it helpful to put our backpack/bags while we walked. Sometimes we parked the jogger for hours and just walked the parks too. We never regretted having it with us. In fact, I was sad when we finally stopped taking it! Every family is different, though, so it's just a matter of finding out what makes your family happiest at WDW. Good luck!