As was shown in the movie Interstellar, when you go close to a black hole or move near the speed of light in space, then your clock runs slower than a clock on Earth.
In the movie, Matthew McConaughey's character was younger than his daughter because he was moving in space.
Question: Even a person moving in space near the speed of light, but his body clock moves to same rate compare to earth. How could travelling near the speed of light keep you young? The only effect you can see is demonstrated with time and a clock, but how could it effect a person's body?