the rule about being 18 is not an actual rule, it's just an extremely clever way to make the back seat seem scarier. They really only check the height requirements. They might ask, "Are you sure you can handle it?" to scare you a bit more but it works for me. When I went on Cyclops the other year, on the car ride there I convinced my girlfriend that it was an actual rule becuase of the physical damage you sustain from the back seat, and she refused to ride the back seat once she saw the sign until the ride op and I convinced her it was just there to scare people. It really works!
-djansi