A and B are twins on the earth, they left the earth on the same day, and A and B were exactly the same young when they left.

A and B have exactly the same acceleration and deceleration process, but A returns to the earth after reaching a speed of 0.1 c. B maintains a constant speed after reaching 0.1 c, and flies forward for another 10 ly, and then returns to the earth . May I ask, when A and B meet again on the earth, which one will be younger?

A and B have exactly the same acceleration and deceleration process, the only difference is that B flies 20 ly more than A at a constant speed. Do you still have questions about the scene I proposed now? If there is no doubt, please tell me when A and B meet, which one is younger?

The above is the question I asked, and James R gave a clear answer: "I think that in this case they will be the same age when they meet up at the end."

Judging from James R's answer, he was very clear about my question and had no objection to the scene I proposed.

Then according to James R, we can draw the conclusion that acceleration is the key factor affecting time dilation. Because A and B have exactly the same acceleration and deceleration process, the only difference is that B flies 20 ly more than A at a constant speed. But James R has clearly told us "I think that in this case they will be the same age when they meet up at the end.” . This means that the difference in B relative to A does not cause B's time to become slower than A's.

B will be younger than A when they meet up again.

From the Earth Frame:

A and B travel outward, accelerating until they reach a speed of 0.1c relative to the Earth. Since you did not specify, to keep things simple, I will assume an extremely high value of acceleration, so that neither craft is far from the Earth upon reaching 0.1 c. This will allow us to avoid the headaches of some complicated math by making the time differences between Earth and ships during the acceleration( as measured from an Inertial frame) insignificant to the scenario as a whole.

IOW, according to Earth, A is gone for only an extremely short period of time and returns having accumulating just a tiny bit less time on its clock. So small, that we can effectively say it is 0. From this point on, A's clock ticks at the same rate as the Earth clock.

B continues on for 10 ly at 0.1c, taking 100 yrs by the Earth's clocks to do so. During which time, B clock accumulates ~99.5 years. B turns around and returns,taking another 100 years to do so, and accumulating another 99.5 years on its clock. B returns to Earth having aged ~1 year less than both Earth and A, have aged 200 yrs.

According to A:

As it accelerates away from Earth, Earth's clocks tick just a tiny bit slower than its own. Now here is where A's acceleration comes into play. Because A is in an accelerated frame of reference, this adds an additional component to what it determines as happening to clocks. It no longer can just consider its speed relative to other clocks, but what direction they are in, how far away they are, and the magnitude of the acceleration. Clocks in the direction of the acceleration run fast by a factor that increases with distance and acceleration magnitude. and clock in the opposite direction run slow by a factor in the same way. Since we set up things so that the distance between A and Earth is this only adds a tiny difference.

While A is accelerating away from Earth, the Earth clock ticks even slower according to A, but when A brakes and then accelerates back to Earth, it means that according to A, during that period, Earth clocks

*run fast* by a tiny bit, so that by the time the two join up again the Earth will have aged a bit more than A. (during the turn-around B will be determined to run slow, but again due to the extremely small change in distance during this period, the accumulated difference will be tiny)

According to B: During the initial acceleration, A's clock runs the same speed as B, and Earth clocks run slow, accumulating just a tiny bit less time. Once it enter the coasting stage, and A's relative velocity increases, A's clocks starting ticking slow, and since A will eventually reach a speed of -.198c relative to B, it will for some time tick slower than Earth's clocks, and end up accumulating just a tiny bit less time than the Earth clock upon returning to Earth.

B continues to coast for 99.5 yrs by its clock, during which time, just over 99 yrs pass on Earth and A's clock due to the relative velocity.

B starts its turnaround acceleration. At this point B is light-years from Earth and A, so this becomes a huge factor in what B determines happens to Earth's and A's clock. So even though the acceleration last for an very short time, the magnitude of that acceleration combined with the distance has has the Earth/A clocks run very fast and accumulate ~2 years in time.

B starts a new coasting phase, taking 99.5 yrs to get back, during which time the Earth/A clocks run slow and accumulate an additional 99 yrs, for a total of 200 yrs for the 199 yrs it took according to B.

When is it said that acceleration is important in these scenarios, it isn't because it has any effect on clocks being accelerated as measured from an inertial frame other than the change in speed, it is because it how the behavior of clocks are measured from within an accelerated frame, and how this causes the observer that undergoes the acceleration to come to the same end conclusion as those that didn't.