First of all, if you subscribes to relativity theory, things with mass can only approach the speed of light, can never be equal or exceeds it even both things are traveling in opposite direction.

Anyway, if you are referring to the twin paradox, where a sister (A) stays on earth and another (B) travel in a spaceship in near c speed, which one will stay younger? The answer is the one in spaceship.

From velocity perspective, if B is moving away from (A) by using A as a reference point, it is equal to A is moving away from B in opposite direction by using B as a reference point, why B is younger? Why not A? Is that your question?

I believe it is something to do with the accceralation impact and also the impact on the biological clock that B is experiencing as she is traveling in high speed.

Staying younger means the total elapsed time experienced by B is shorter than A when they meet up again. If 10 years has passed for A on earth, B might only experienced 7 years traveling.

If B and A carried a digital date and clock system with them, the date and time of B would be shorter than A.

B is younger as she was within the timewarp environment that was different compared to A.

Here is one extract from wiki:

If we placed a living organism in a box ... one could arrange that the organism, after any arbitrary lengthy flight, could be returned to its original spot in a scarcely altered condition, while corresponding organisms which had remained in their original positions had already long since given way to new generations. For the moving organism the lengthy time of the journey was a mere instant, provided the motion took place with approximately the speed of light. (in Resnick and Halliday, 1992)

On the twin paradox subject, many forumer suggested that both the twin should aged the same when B touch down on earth as aging is a biological process and both of them should experiencing the similar molecules changes regardless of the velocity and other factor differences. Both should aged 40 years old and not one (A) 40 years and (B) 28 years.

Just like you, it also feel strange when I first tried to understand the special theory (SR) and general theory (GR) of relativity. Both theories defy the common sense we known for years and tells us a different phenomenons on how we perceive things.

The theories indeed suggests that B, the traveler aged slower (hence, younger) than A on earth due to the following:

1) The blue/red shift (like doppler effect) of light on both of the reference frames
2) Time dilation factor - The faster the speed closer to c, the higher the effect
3) Acceleration effect experienced by B in the ship while A was stationary on earth
4) The effect of gravity. Gravity has confirmed slowing down the time. When B is in a ship, the acceleration/deceleration effect on him was like a large impact of gravitational forces acted on him
5) Space/Time compression factor

An experiment was actually been done which was to start off with two identical, synchronised atomic clocks, keep one on the ground and fly the other one around in a plane for a while. When the plane lands, the clock that was in the plane is found to have run a little slower than the one on the ground.

Still not convince enough, ask this question, what do you mean by both A and B should be aged the same, ie the grow rate are the same?

The word rate means that it is time dependent. When traveling near light speed, time is no longer like what we perceive at low speed. The near light speed experience alter the space time continuum. One needs to throw away the idea there is only one "absolute" time idea in order to appreciate this.

If you are interested to know more, here is one of the link:

http://www.gmarts.org/index.php?go=420

Summary
Bob and the rocket have aged only 12 years while Ann and the Earth have aged 20 years.

The twins' times are different when Bob returns because Ann has seen Bob's time running slower than her own on average, while Bob in the rocket has seen Earth time running faster than his own on average.

Cheers.

Not quite true. Physicists leave this possibility open.

You have a point on thing happened cannot be unhappened. This is called cause and effect casual loop. Changing the past is paradoxical, however affecting the past is logically feasible.

Kurt Godel (who is more famous for his Godel's incompleteness theorems), a colleague of Einstein, came out with the following comments after going through a lot of research on Einstein gravitation field equation:

"By making a round trip on a rocket ship in a sufficiently wide course, it is possible to travel into any region of the past, present and future".

Why so? The reason is past, "Now" and future are merely an illusion. There is no absolute "Now" per say. Just look at the twin, for Ann who is station on earth, her time that she "feel NOW" is 2010. For twin Betty who is traveling near light speed, her time that she "feel" could be 2005 and shed aged 5 years less that Ann.

For a population of living organism on Halley Comet which travel in high speed, the time "Now" that they feel could be 300BC (earth time) and they might be observing the event happening in 300BC now.

The reality is, there is no such thing as "Now". It is just a misinterpretation on our part on tracking the events around us in this spacetime continuum.

The following is a snapshot from the discussion under a different topic "time - discussion".

If you do not believe, try this. Say "Now". By the time you say "Now", you assume that the "Now" you just said is gone and you have to repeat saying "Now". Again, it is gone, and you have to keep repeating saying "Now" again.

You assume that every "Now" you mentioned has "flown" through time and become the "Past". And you assume that every moment is "Now". If every moment is now, what do you mean by tomorrow? You have to agree that tomorrow will never come.

If you are interested, you could go to the time topic for more explanation and discussion.

Have a wonderful new year ahead!