It should be based on size...not age. As long as a child fits into the stroller, what's the difference how old they are? I know even my 11 y/o would have liked to stroll for a while...
Also...my 3-1/2 year old would have ended up wanting to be carried on daddy's shoulders if we hadn't of had the stroller for him.
Disney is alot of walking and I think it should be made as easy as possible for the kids. After all, you want everyone to have as much fun as possible...if the kids are crabby and tired because they're legs hurt, that isn't much fun for them