As a Tolkien fan I have two models -
elves who reach that middle life stage and never get the problems of aging. They also differ from mortals in not growing bored or tired of the world, so the unchanging eternity of their lifespan isn’t a problem as long as the surroundings match. Athrabeth goes further with the absence of Platonic Ideals that set them apart from how mortals see the world. Then Númenoreans, which are closer to the literary fantasy tropes the Ancients are dealing with. Still human with extended lifespan and magically heightened immune systems and the ability to retain the middle age vigor for about two centuries until the symptoms of old age set in rapidly at the last decade. At which point the ‘High Men’ could willingly relinquish their lives and die peacefully of their own volition. Sounds familiar. Of course they live in a paradise terraformed just for them but said paradise is within spitting distance of Elf Paradise with ‘immortality until the planet dies’ - naturally jealousy sets in, death is no longer peacefully accepted but avoided until dementia and sickness weaken it to unavoidably. Civil War, Atlantis. And ugly fandom debates on which side was right and tossing around accusations of euthanasia that are the equal to XIV’s Sundering Debate.
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)
In seriousness, Ancient biology wasn’t a match for mortal biology even if the aether soul density was equal, but there’s probably an aether usage to retain youth that you don’t bother with if you’re an old enough Ancient that everyone knows you’d just being vain or uncomfortably self-conscious about your looks changing to keep it up.