Sorry, I know there are tons of threads about this, but I've been though them all and couldn't answer this satisfactorily myself. You know how it goes. You have 2 twins, A and B, who are magically the same age exactly. They start at the same point in space, each in their own spaceship. They then accelerate at the same rate until reaching the same constant velocity, somewhere close to the speed of light. After some time, A decelerates and stops. B continues at the same near-light velocity for some time longer, then decelerates in exactly the same manner as A and stops. Both twins have undergone the same acceleration and deceleration. The only difference was that B stayed at a high velocity for longer. Is one twin older than the other?