Alright I promised I'd do a proper theory post so here it is
~ On The Lifespan And Development Of Long-Life Species ~
☆ Xianzhou native humans:
Can live for thousands of years and do not die of old age, rather eventually succumb to mara.
My theory is that their physical development is not directly proportional to a regular human's but slows exponentially: as young children they grow at a typical human rate, then progressively more slowly over time.
Yanqing in the new event estimates himself to be the same age as Luka, who appears as a late teen or young adult, so from this we can gather that by around 18-20 a Xianzhou long-life human has equivalent physical maturity to a young teen. We also know thanks to Huaiyan that they do (or at least, can) continue to age past physical maturity. While we don't really know for sure what's going on with Huaiyan specifically we have been told that he is the oldest of the Arbiter-Generals, and potentially many times older than everyone else we've seen, so it's not a huge leap to assume Xianzhou natives just age extremely slowly past adulthood and most become mara-struck long before they show signs of advanced age.
Culturally, it seems the age of adulthood for Xianzhou humans is variously considered to be 100-200 years old, which also supports this. I do see that as predominantly a social consequence of their longevity and lifestyle, and indeed we know they tend to appear as adults well before the age of 100, though there are suggestions that continuing to grow taller beyond that age is plausible. My impression is that it's typical to spend up to a century or maybe even longer hovering in that late adolescent almost-fully-grown state, by which point physical development has slowed to a crawl.
Adding Xianzhou cultural notions of what constitutes cognitive and emotional maturity (the mental hallmarks of having existed for hundreds of years) = they spend a long time functionally adults but not quite regarded as such by their society.
By contrast, it seems the Vidyadhara typically age at a rate linearly comparable to short-life humans, just much slower, with physical maturity taking 100-200 years to reach and maximum lifespan being around 700 before they must reincarnate. Though we've seen few examples of this (actually I don't know if there have been any in the game itself, though Dan Heng alludes to it in the 2.5 story) the Vidyadhara do physically grow old as they approach this mark.
Complicating things, it's becoming more common over time for Vidyadhara to have stunted physical growth and spend centuries or even their whole lives with the appearance of children - this seems to be purely physical. Bailu is a special case here as it's hinted that her development is being intentionally blocked by the device on her tail.
High Elders are implied to perhaps also be a special case, though this should be taken with a grain of salt as the evidence here is a comment in the context of an exaggerated story as well as one of Dan Heng's Imbibitor Lunae eidolon pictures. It's possible that they are reborn already being mostly or fully physically mature and do not age normally as a result of inheriting the High Elder powers.
Interactions with the Vidyadhara eggs in Scalegorge Waterscape seem to imply that a Vidyadhara can potentially spend several centuries within an egg before being reborn, as some of the inhabitants of the eggs are suggested to have entered hatching rebirth shortly after the Sedition of Imbibitor Lunae and still have yet to hatch, though it's possible these memory impressions are from previous incarnations.
Foxians are the shortest-lived of the "long-life" Xianzhou inhabitants, with a maximum lifespan between 250-450 years. Notably, they appear physically youthful until almost the end of their natural lives, aging rapidly in their final few years.
We don't have much direct evidence regarding how quickly they grow as children, though at a guess I would say the same as short-life humans given their mechanism of longevity is stated to be "an abundance of powerful stem cells" which doesn't sound like it would slow their development so much as just prevent them from age-related degeneration.
