Ancient lore from Korean liveletter Q&A

[Lurina]

We intuited from Emet's rose-tinted view that the Ancients kinda reached a point and then they stopped. And were immortal. But aging, especially past a "prime" is a big point of mortality, and that's not even getting into the multiple myths about people who attained immortality without the eternal youth as a required secondary superpower and... it's not pretty. Did Ancients have to deal with osteoporosis? incontinence? Senility? And while Lahabrea isn't an OLD man, he IS grey, he has wrinkles, and he's still "young" enough to be in charge of Pandaemonium and hold a seat at the Convocation. While Venat may be retired, it also claims she's younger than him (and her white hair kinda masks if she were a woman of a certain age)...

My take is, eventually, after a very, very, very long time, the potency of their aether would begin to wane, and they would start to age in a manner that would affect their bodies and how they look, akin to mortals. I think that it would take an exceptional amount of time to approach the levels of aging that you describe, though, and mentally they would have been prepared, and willingly chosen to, return to the star long before that point, having likely achieved their life's purpose (whatsoever that may be) in the course of their already enormous lifespan.

Sorry, this might be the fault of my translation, but to be clear, the quote is saying they can choose not to cosmetically (as well as meaningfully, I think?) age using creation magic in their modern era. The exact phrasing is "someone (using creation magic) may maintain their youth enough to be mistaken for a young person".

Unless this “their outward appearance of aging is tied to their self image” implies that their physical bodies are malleable in ways beyond that of aging and could include gender and appearance. (And which point the Ancients are the Valar but of course they were)

There's a quote from Themis in 6.2 where you're discussing the possibility of someone disguising themselves as Lahabrea where he says that the Ancients are able to "control their forms to a limited extent", so it's probably that.