In the movie.

It makes sense to explain that they are still on the same timeline. When they returned from the water planet, Cooper's daughter was as big as Cooper because of the gravity of the b

It makes sense to explain that they are still on the same timeline. When they returned from the water planet, Cooper's daughter was as big as Cooper because of the gravity of the black hole, and then they went to Mann's planet, and then they entered or approached the black hole (Cooper entered and Brand approached the black hole). This kind of tossing will make their time slower than on the earth, even much slower than on the water planet. This can fully show that although Cooper and Brand don't have much time here, Murphy on the earth is old, and Cooper and Brand are both young at both ends of the wormhole.