Why Johnathan didn't mention NIMH to Mrs. B.

Okay, so I just watched the movie for the first time last night, and I thought it was awesome. Not quite what I expected since I thought the escape from NIMH would be the focus, but, either way, a great film.

I just have a question about the scene where Mrs. Brisby is talking to Nicodemus about her husband and the escape. Mrs. B. wonders aloud to Nicodemus why her husband didn't mention anything about NIMH to her while he was still alive, to which Nicodemus responds about the aging process the injections affected (that she would grow old while he remained young). I've never read the book, so I wondered if anyone here could clear up why that would be the response Nicodemus provided. Does that fact that Johnathan would remain young play a greater significance in the novel? Is there more to the fact than the film provides?

Why did I write? Because I found life unsatisfactory.
*~Tennessee Williams~*