Ok so, supposedly you have two twins in [seperate] spaceships, one approaches the speed of light going away from his twin, then decelerates until going the same high speed but opposite direction, decelerates again to stop next to his twin buddy. The twin that did not move (relative to the very initial reference frame) is older than the twin who approached speed of light. By symmetry, this time dilation MUST have occurred during the acceleration of the twin that moved because any time not during the acceleration, each saw the other moving at near the speed of light. The twins start off at the same age (approximately... ) and end up with very different ages. Therefor the time dilation must have occurred during the only asymmetry, which is when one twin felt an 'inertial drag' so to speak. (and the other twin never felt this) Why is the time dilation a function of velocity then? Shouldn't it have to do with acceleration?