I think you left out some other essential notes:

1.) End of the World isn't consistently young in the flashback in Episode 34. He ages visibly (and, if I recall, his voice changes) during his conversation with Utena. Here he is just after the kiss.



2.) Akio is explicitly presented as not having aged between Nemuro's time and the series's "present."

3.) Flashback End of the World doesn't relate the Tale of the Rose as if it happened recently; instead, he emphasizes the eternal nature of the Rose Bride's suffering. Likewise, in the flashback, the time signals are all mixed up, with a fax machine and men in suits alongside very medieval imagery of huts, witches, and swords.

4.) Maybe it's just the expression or the lighting, but Anthy seems to be drawn as significantly older a couple times, when her power and presence are emphasized. For instance, here she is with her hair down lying next to Utena, looking relatively young, and then standing next to Akio:




For these reasons (mostly one and two), I've always taken their ages in flashbacks to be subjective, a visual device used in the story that's not intended to mirror the absolute passage of time. I expect they appear to be the ages they are in the series because they intend to.

That said, it does seem like we're definitely supposed to equate the Tale of the Rose era with their "youth," and the aging of Dios/Akio in particular seems to be a significant metaphor. If Akio has chosen the role and appearance of an "adult," that's even more significant in terms to his character than if it was an involuntary transition.

To me it's always been clear that these two are of whatever age that suits them the best for any given situation. Utena first sees Dios as a child, since as a child herself, she empathises with this form the best, but as she begins to look up to him as their conversation progresses, he becomes older and easier to idolize. Anthy is shown capable of being anything from unassuming geek to otherworldly seductress to a lost child, as her moods and the situation she tries to adapt to take her.

My personal theory of the siblings is that they are archetypical beings, a bit like Neil Gaiman's Endless, but gotten corrupted and muddled, leaving the original ideas that they represent hazy and unclear. They still have some remnant of the Yin-Yang distinction that I think would represent their two sides at their purest: Dios as pro-active, positive, masculine, and Anthy as passive, negative, feminine. Ironically, in the end Anthy may have won in the end, becoming an active mistress of her own fate, while Akio ends up doomed in obscurity, playing the same pattern forever, never managing to break loose. There was a scene in one episode where Akio lays over Anthy, and his body is filled with stars, while she's solid and (at least apparently) passively receptive. I've mentioned this before, but it reminds me of the old sky god-earth goddess dualities. Still, I think that Yin and Yang are the best representatives of what the two are supposed to be, as opposed to the Western Pagan or Christian dualities.

Anthy is often catagorized as a witch. Since witches mythologically are associated with illusions, I always assumed that Anthy had magical illusion powers to appear whatever age she needed to be, depending on the situation.

Akio is less easy to describe this way, except he has the powers of the observatory, which may give him some illusionary powers as well.

Age really is an illusion to them, they have a very immortal quality. I imagine they appear as they need to.

Giovanna wrote:

hollow_rose wrote:

I imagine they appear as they need to.

Which raises an interesting side note to their deciding their age. They also (presumably) decide how they look--dark skinned and very different from everyone else. Would it not benefit them to blend absolutely? Or do people really not notice the difference? (Is it ever even really addressed that they appear a totally different race?) It seems almost like stubborn pride that they keep their coloring

This is an anime thing in general, though.  Sailor moon isn't blond, she's just drawn that way.  Japanese characters don't have blue hair or pink hair or whatever, they are just drawn as if they do.  General, the authors avoid referencing hair color, unless there's a foreign character who can actually have blond or red hair.