We were there two weeks ago. Our day and a half was not enough time to do everything. Of course, we were two adults in our 50s and we actually read all the exhibit labels, went on the rocket garden walking tour and stayed after to ask questions of the guide, saw both IMAX films and the Star Trek show, attended the astronaut lecture, went through the robot explorers show/exhibit, and took the included bus tour. We never made it to the Astronaut Hall of Fame (included in the admission) which has more intensive shuttle experiences.
This is not a theme park with a lot of hands-on activities. It's educational. Our sons would have enjoyed it at age 8 and up but they were early readers and interested in science and the space program. They had been to the NASA Goddard Space Flight Center and Smithsonian Air and Space Museum by about that age as well as being Star Trek and Star Wars fans along with us.