We've done it twice. Once when DS was 4, and again when he was 7. 4 was too young - while he had fun he didn't really get it. 7 was much better, he'll be 10 for our next trip and I think he'd enjoy it at 10 (though we've done it twice - so won't do it again). I'd say 6 or 7 to about 10 or 11 is the right age.
We had two different tours - though my understanding is that there are only two versions of the tour - so we've done them both.
Have fun!