CMIF simultaneity says the accelerating observer (he) MUST adopt the conclusion (about the current age of the distant person) of the perpetually-inertial observer (PIO) who is co-located and stationary with respect to him at that instant in the his life. I.e., if that PIO concludes that the distant person is currently T years old "right then", then the accelerating observer must adopt that conclusion.

Yes, so using v=+0.866c, if the traveling twin (he) is 20 years old, he would say his stay-home twin sister (she) is 10 years old, and if at that point he jumps off the train he was riding so v=0.000c now, he will have to re-calculate and say that she is 40 years old. That is because all of her grid clocks are stationary on the ground and they are all synchronised to her age, and they are all displaying 40 years at that time, simultaneously. He has no choice but to agree that she must be 40 when he is stationary with respect to those grid clocks, because in that case they are synchronised and all displaying 40 at the same time.

Of course if he then jumps back on the train so v=+0.866c again (in the same direction as before), then he must recalculate again and say she is 10 years old again. He would not want to keep referring to those other clocks that he is no longer stationary with respect to, because when he is moving with respect to them they are not synchronised. Instead he would have to refer to the clocks on the train which are synchronised to each other, and all displaying his age 20 simultaeously in the train frame. Then he would have to figure out her age based on its relationship to the train clocks, and he figures she must be 10. And someday he will find proof that he was correct, because there can be a photograph of her celebrating her 10th birthday while a train clock displaying 20 happens to be passing her by.

Let's look carefully at what happened. The slope of the traveler's line of simultaneity changed twice. First it pointed to her being 10, then it pointed to her being 40, and then it pointed to her being 10 again. You can choose to call that "negative aging", but we know she does not age negatively in her own fame, or in any inertial reference frame, so the actual meaning of that statement is highly questionable.

But my question for you, Neddy, is this: Do YOU subscribe to CMIF simultaneity? I always thought you did, but I think lately, at least, you've been in the "no negative ageing" camp ... i.e., I think you hold that the accelerating observer will never conclude that the distant person is abruptly getting YOUNGER. But that is exactly what CMIF simultaneity says can happen. If you disagree with that last statement, let me know, and we can discuss some Minkowski diagrams.

There is no problem with CMIF simultaneity. It never produces any absurd result such as your method does. Namely, in the example above using v=+0.866c:

If the traveler is 20 years old and says his twin sister is 10 years old, and if at that point he jumps off the train so v=0.000c, your method would have him still saying that she is 10 years old (although aging faster now) instead of simply recalculating her age to be 40.

This means that you have one inertial person (him) claiming she is

**10**, even though he is stationary in an inertial frame where all of the frame clocks synchronised to her age are all simultaneously displaying

**40** years, including one that might be located right next to him.

You want physicists to believe that SR was never clear about whether or not she is actually 40 years old in her own frame when all of those frame clocks synchronised to her age are simultaneously displaying 40. You think maybe SR was saying that she can be both 10 and 40 at the same time, in her own frame, which is absurd. You are attempting to make a mockery of SR and the physicists who understand it.