Jonathan couldn't tell you about NIMH because the injections slowed the aging process.
7 seconds sound clip from the The Secret of NIMH (1982) movie soundboard.
You can hear this line at 00:54:57.848 in the Blu-ray version of the movie.
Quote context
[...]
- When worn by one with a courageous heart, the stone glows red. It becomes a blinding radiance.
- Courage of the heart is very rare. The stone has a power when it's there.
- You can unlock any door if you only have the key.
- Oh, thank you. I will treasure it always.
- Jonathan couldn't tell you about NIMH because the injections slowed the aging process.
- You see, you would've grown old while he remained young.
- Nicodemus. The boats are ready and waiting. We can leave any time.
- Good.
- My child, shall we journey to your home?
- Sir?
- Tonight you must move your house to the protected side of the stone.
- Yes.
[...]