Okay, I have an issue here. Why is the twin aging slowly on the return trip? It doesn't make sense to me, and I'll explain my position:
Take a distant planet, say, 500 light years away. If an observer were to travel to that planet (no return trip), how could the planet age slowly on the trip there and yet become so aged in the slow down? I don't really get it.
For clarity, I'll post part of Janus' original statement to help explain why.
The Earth twin ages slowly on the trip out, ages a lot very rapidly during turn around, and then ages slowly again on the trip back,
The twin is undergoing acceleration (or, in this case, deceleration) during turn around which obviously causes the time dilation to decrease, thereby meaning that the twin will start to age more rapidly.
By slowly, I believe Janus is referring to the fact that the twin is moving at a constant velocity, so the time dilation effect will also be constant, meaning that the twin will indeed age slowly.
Admittedly, I am a bit confused by Janus. Why would the Earth twin age slowly and appear to age more rapidly, when the effect of time dilation is actually felt by the twin in the ship?
However, I've answered your question, Arcane, without referring to the twin on Earth or space, so that the answer can then apply to what Janus replies.