I think 5 and 4 are the perfect ages for this! My DD was 4 years, 3 months old when we did it, and she LOVED it. She totally bought into it. Not to give too much of a spoiler, but they start the tour under one premise and then an "urgent message" arrives, changing the tour into more of a "mission." DD kept trying to get other Guests (who weren't on the tour) involved; she was so concerned about achieving our goal!
We will go again when DS is between 4 and 5. DD will be between 7 and 8 then, so it should still be fun for her, but I doubt she'll buy into the premise as hook, line, and sinker as she did at 4. I can't wait to share the experience with DS, who napped in his stroller and missed all of it the first time around!