I don't think there needs to be a level between Casey Jr and BTM or Crush. My kids went on BTM from age 4/5 and Crush from age 6.
Last year we were there when they were 8 and 10 and they enjoyed ToT, Crush, BTM, Space Mountain, RnR coaster etc etc. My DD (

was not tall enough for Space Mountain but she's the one that went on RC Racer on her own because the rest of us were too chicken!
Going back next month and they're looking to do Indiana Jones as we have never done that one.. Each time we go they are a little older and there's always something new to try.