Others believe that the the current age of a distant person, according to the traveler, is some definite value that is "non-negotiable" and non-discretionary. Among this group, some believe that that definite value can only be properly determined by using the general theory of relativity, via the equivalence principle. Conversely, others believe that, in the (assumed) absence of any significant masses within the spatial region of interest, that the special theory of relativity is all that is needed to provide that definite current age of the distant person. But, even among this latter group, there is disagreement as to WHAT that definite value of the current age IS.

I am personally in the above quoted latter group (traveler's perspective is definite and non-negotiable, and determinable purely via special relativity).

There are two reasons why I don't believe that the traveler's perspective is arbitrary, "negotiable", and/or "meaningless". The first reason is philosophical: IF the home twin (she) exists AT ALL at some given instant of the traveler's (his) life when he is distant from her, then it would seem that she must be doing SOMETHING definite at that instant of his life. And that SOMETHING defines a unique instant in HER life.

The second reason is that the simultaneity result given by the Lorentz equations isn't just some meaningless age: it is the result that the traveler can determine from his own elementary calculations, when he receives a sequence of radio messages from her, giving her current age at the time she transmits the message. If he properly allows for her ageing during the transit of the message, and adds that to her announced age in her message (when she TRANSMITTED that message), in order to get her age when he RECEIVES that message, then he will get the same result given by the Lorentz equations. I show the process that the traveler must use, to properly determine her ageing during the transit of her message, in my paper

"Accelerated Observers in Special Relativity", PHYSICS ESSAYS, December 1999, p629.

The above comments apply to the case where each twin is perpetually inertial (i.e., neither of them ever accelerate), so that there is no doubt that each twin is entitled to use the Lorentz equations (and the time-dilation result). But I also show, in the above paper, that regardless of how the traveler accelerates, that his conclusions (at each instant of his life) about the current age of the home twin, is ALWAYS exactly the same as the conclusion of a perpetually-inertial person who happens to be momentarily co-located and mutually-stationary with the traveler at that instant. That means that the traveler himself is entitled to use the Lorentz equations to determine simultaneity at each instant of his life, whether he is accelerating then or not. I call that momentarily-stationary inertial reference frame the "MSIRF". (It is usually referred to by others as "the co-moving inertial frame", but I prefer my term). This same method of obtaining the traveler's perspective, of the current age of the home twin, has been used by others. For example, Taylor and Wheeler used the same approach in their example (Example 49) in their "Spacetime Physics" book, pp. 94-95. And Brian Greene in his NOVA series (and in his book) on the "Fabric of the Cosmos" also used the same approach.

The simultaneity result given by the Lorentz equations can also be determined by an equation I derived long ago that I call "the CADO equation". I originally coined the acronym "CADO" just to save time having to repeatedly write the phrase " the Current Age of a Distant Object". The CADO equation gives exactly the same answer as is given by the Lorentz equations (as it must, since it was derived from the Lorentz equations) ... it's just easier and faster than using the Lorentz equations, and it is less likely to be miss-used in applications. The CADO equation is especially simple and quick to use when all the velocity changes are the idealized instantaneous changes, but it is also fairly easy to use for piecewise-constant finite accelerations (perhaps alternating with "coasting" segments with no acceleration). It is even valid for any acceleration profile whatsoever, although in continuously-varying-acceleration scenarios, numerical integration will generally be required. The CADO equation is fully explained and illustrated on the webpage

newbielink:https://sites.google.com/site/cadoequation/cado-reference-frame

[nonactive] and its derivation (as well as a good bit of additional information) are given in my previously referenced paper.