> That's relativity, folks
quote from the movie
Cooper doesn't age because he went into the black hole, meanwhile time passed quicker on earth or elsewhere.
From his point of view time passed 'normally'.
Anyhow still movie makers don't understand much more than us so I wouldn't rely on that.
It's unclear how Brandt remained the same age, she was affected by the black hole too but less since she escaped it to get to the third planet.
Maybe she hybernated herself, who knows.
I've read somewhere we get easily fooled because in this universe the 4th dimension is extremly close to 0, whatever that means.
Also lookup anisotropic universe, isotropy is at the base of our mainstream understanding and it's an assumption.
And probably wrong.