travelling twin keeps going in a straight line at constant speed, at the point at which he is 10ly from Earth, is he younger than the other twin? There may be no way he could know that for sure, but the age difference should be there, unless we are saying that the age difference exists only if it can be directly observed.
Hmm. If the travelling one stopped when he got to 10ly, then we could know for sure because they'd both be in the same reference frame. One way is to have clocks strewn about the universe, all stationary in the Earth's reference frame, then no matter how far away you are, you can tell the time. Not that I have an answer, but it'd be nice to figure one out.
If the travelling one didn't stop, then I'm pretty sure the people on Earth, looking through their telescopes and accounting for light travel time, would see him as being younger than the twin on Earth. But here the situation may well be symmetrical, with the travelling one also seeing the Earthy one being younger.
