This question has been asked before in the form of the 'Twin paradox' , and there are 42 pages of questions on this site alone when I search for 'twin paradox'.
The counterargument has been 'that the traveling twin has to accelerate and therefore is not in an inertial frame of reference.'
OK, I believe that; it is consistent with the lay explanation of general relativity. This effect of gravitational time dilation was dramatically portrayed in the film 'Interstellar' when the crew goes to a planet near a black hole and everyone else gets a lot older.
So what happens if we use the 'equivalence principle' and provide a gravitational acceleration to the non traveling twin? I have read many scenarios described on this site where the observer will not be able to tell if he is on earth or on the accelerating spaceship, yet the many of the questions here on physics stack exchange are answered with the statement 'the spaceship traveler will age more slowly'.
If I can slow aging by 'dialing up gravity', or I can slow aging by traveling fast, why is there always the statement that 'the spaceship traveler will age more slowly'? Why does 1 g on a spaceship age you more slowly than 1 g on earth?