I can't comment on the celebrity status, or what the typically age restricts on international travel are in AUS/NZL, but I know you can't do many things in the USA and a lot of Western Hemisphere nations unless you are at least 20. There's probably also a lot of legal things that go into the planning with all the different regulations that require you to be 21 (i.e. waivers and stuff, no?)
Just speculating - GOOD LUCK if anyone from down under is submitting!! Watching season 2 only reaffirmed my LOVE of Australian accents and culture. Can't get enough of that sheer positivity, Americans need more of it.