You can't really impose age restrictions for rides like that at an amusement park. First, you'd have to carry around your birth certificate or something everywhere since younger children have no immediately available identifcation info such as driver's licenses, etc. That would just be a hassle, not to mention how much longer it would take for everyones' ID to be checked and verified. o_O Besides, it's not like age really matters -- you can be twenty and still be shorter than a thirteen-year-old, but that doesn't mean the elder person will be allowed on whereas the younger isn't. If you're below a certain height, then you can easily be thrown from or tossed around on a ride; that's why the restriction is there. It's got nothing to do with maturity and everything to do with safety.
Jon ~ They weren't arrested, he was just counteracting a point made by someone else who said they should have been arrested. :O