The crew I was working on had people from like 16 (officially, there were a few kids of con workers that were a bit younger helping out as well) to as old as I think mid-70s, Kip said. So, not sure about all US cons, but at least Showmasters (which is what I was working for) takes on all ages! ^_^

And I do recommend it if you get the chance, it is highly entertaining! ^__^