So say that there are twins. Twin A is near a heavy planet and twin B is out far away in space. Why does time slow for twin A compared to B? Can't they say that they both are in inertial frames because there are no forces(gravity isn't a force) acting upon them? Therefore for each of them can consider the other younger. After all, twin A can just say that he is stationary and that its B that is under a high gravitational field and is accelerating and vice versa.