There are 2 twins born at the same time. One goes out into space in a theoretical rocketship that can nearly reach the speed of light and one stays on earth. Time dilation occurs and the time on the rocket seems to pass more slowly than on earth. On return to earth, depending on the speed of the rocket and time of the trip, the twin in space would be younger than the one on earth. But how can two twins, born at the same time be different ages?