Have you see the cartoon adaptation, "The Secret of NIMH" ? I practically lived off that Betamax tape for the better part of my toddler years. The only difference between the source material and the film is that they changed Mrs. Frisby to Mrs. Brisby (maybe to avoid any allusions to a flying disc), but otherwise, Nicodemus, Justin, Jeremy the Crow, and Mr. Ages are all there!
I never really knew too many outside my family who knew this story of NIMH... cool!