This image should illustrate my point.
The red A is the path through time of a stationary object. The red B is the path through time that another object takes if it is moving very fast through space. You can see that red A will reach the red line first (red line = a specific moment in time)
Then we turn the diagram slightly so that red B (now shown in black) is now going straight. Remember all we have changed is the point of view from which we are looking at it. From this second point of view we can see that now red (now black) A is moving more slowly through time relative to B.
Basically, it all depends on which one you look at or measure. But what happens if you don't measure either? I have a hard time believing the accepted view that they are in a superposition of states - both ahead and both behind in time untill someone chooses one to measure.