Again, you're talking about the Twin Paradox, which is only applicable to General Relativity. Newtype is talking about Special Relativity. :bitch:
So fricken annoying to see the two so constantly conflated, which is why I despise the twin paradox.
No, the twin paradox comes from special
relativity, which had no trouble explaining that the twins wouldn't be the same age, just trouble explaining why A should be older B instead of B older than A, if there wasn't a preferred reference frame, which special relativity said there wasn't. That's why it was called a paradox
As I recall, Einstein had largely resolved it around 1910, before general relativity, by distinguishing btween the non-accelerated reference frame and the accelerated one, which initially would also seem to be relative (which one is accelerating? Wouldn't it depend on your point of view?), but only one twin feels like he's pulling some serious G's somewhere along the way, leading to the importance of the non-accelerated reference frame's proper time, etc.
But for the twins paradox to occur, one has to change your inertial frame of reference, which is the domain of general
relativity. Otherwise, one twin sees the other as aging more slowly, and vice versa.