The twin Paradox, What if they never meet and they are observed by an outside observer? We have two twins, one on earth and one on a rocket traveling at very very high speed, near the speed of light, away from earth
After some time, the rocket decelerates so that the distance between him and his earth twin is nearly constant.
Suppose a third individual exists always exactly in the middle between the two twins(or belonging to the perpendicular bisector of the fictional line between the two twins), the observer uses a telescope to observe each twin.
So which one of the two twins aged more, the one on the rocket or the one on earth? 
From the earth twin point of view he is stationary while from that of the rocket twin he is one stationary so who aged according to the third individual? Or does this not fall in the case of the twin paradox because they never meet?
 A: 
After some time, the rocket decelerates so that the distance between
  him and his earth twin is nearly constant.

In this case (even though the twins never meet!) it would be possible for them to come to agreement about who is older. F.e. they could start sending messages to each other. As soon as one of them receives message he appends his own age to the end of the message and sends it back. The resulting list can look like:


*

*I am first twin. I am 1 year old now.

*I am second twin. I am 2 year's 1 month old now.

*I am first twin. I am 3 year's old now.

*I am second twin. I am 4 year's 1 month old now.
...


Having analysed the message they can both agree that the second twin is 1 month older than the first one.
Same result would get the third observer who stays in the middle of the (now staying still) twins.
The older twin would be the one who stayed on Earth.
The paradox is that from the point of view of the travelling twin he was staying still and the other twin was actually moving! The situation seems symmetric! Both twins describe the situation like this: I was staying still, the other twin moved away from me very fast, than stopped and now we compare our ages.
Solution of this paradox is that situation IS NOT symmetric. Because the frame of reference the second twin is using is not inertial.
A: The neat thing about special relativity is that you can do the calculations in any reference frame and you will arrive at an answer which is consistent.  In this case, the key is that the information is traveling to this third observer, and they are making all of the decisions.  Thus, it makes sense to solve the problem in the reference frame of this third observer.
As far as the third observer is concerned, the rocket is shooting away from them at 1/2 the velocity that ground observers saw.  Why?  Because he's staying half way between the planet and the rocket, so he's always going to see the rocket having 1/2 the velocity ground observer see.  (Edit: as Lesnik points out, at relatavistic speeds it wont be exactly 1/2 the velocity.  However, it is guaranteed to be exactly the same velocity in each direction, and that's what matters)
He also is going to see a planet rocketing away in the other direction at the same velocity!  The fact that planets are big doesn't matter.  From his frame of reference, the planet is falling away.  This is just like how a skydiver perceives the plane shooting up away from them when a ground observer would say that the plane was flying level and the skydiver was falling.
Thus, we have a very simple situation.  Two twins, traveling away from the third observer at the same velocities, with the same acceleration profiles.  End result: the third observer will see them at exactly the same age.
