Things aren't symetrical because only one of the two underwent acceleration. So one of them was not in an inertial frame of reference. This is a famous paradox

Things aren't symetrical because only one of the two underwent acceleration. So one of them was not in an inertial frame of reference. This is a famous paradox, see here for a full discussion/resolution: https://en.wikipedia.org/wiki/Twin_paradox https://en.wikipedia.org/wiki/Twin_paradox