Let's say someone [me] anyone, got a bit ambitious and made the world lifespan (semi-recorded history) a tad longer than it should probably ever be. For Earth, humanity's "history" of sorts can go back aprox. 10,000 ish years yeah? Times that by-a lot- and problems pop up. Mainly, how do you handle life spans when someone [still me] decides "Hey! With that amount of time there's no way only one sentient race exists" and pooof Way more things than there's a right to all of which probably should have varying life spans cause magic and cause can. So, bottom line, anyone got tips on lifespans they've seen handled well or the really bad ones I can steer away from?
Do you mean an individual's lifespan or lifespan of a species? There are species on Earth right now that go back in the millions of years.
There's a few potential questions in here, so I'll try to answer the ones I can see. First of all, I would make enough backstory to ensure that there's reasons behind the emerging of the different races, while also inserting plot elements that ensure that they ARE or ARE NOT actually different races. The first is something that I naturally do while building a world, coming up with reasons why the different races came into being. Maybe one of them were transformed from a different race to be better slaves, while another evolved on a different continent that was far enough away to avoid the two race's evolution meeting for a long time. A fourth race could have come into existence from experiments in an attempt to become immortal, while a fifth could have become a race of telepathic and telekinetic animals due to mystical energy permuting everything. In these cases, races 1 (initial) & 2 (transformed slave race) would biologically be the same race, while the fourth race (immortality experiment) could be the same biological race as any of the others. And by "same biological race", I mean that they can breed and have fertile children, like different breeds of dog, cat, or the various cultures of humanity. If you have half-elves, half-dwarves and half-orcs that can breed with humans, they are all the same biological race as humans. And, yes, it is possible to have elves and orcs who are the same biological race as humans, but not the same biological race as each other. There's some series of flocks of birds that live around a certain bay I vaguely remember from high-school biology that can breed fine with their neighbouring flocks, but can't with flocks situated too far from them around the bay. The second question I can come up with is, what are the notable differences between the races? Do you do the standard "Beastman race", which covers what could easily be 30+ different appearances without explanation, do you do what One Piece did with the Fish-Men, making it so that their biology is a tangled web of races based on their ancestry allowing for a Clownfish-Man and a Leopard-Shark-Woman to give birth to an Octopus-Child, or do you actually make 30+ different races based on the humanoid animal? Is temporary shape-shifting a thing, and does it change the biology of the person using it sufficiently to breed beyond races? That could even be a reason behind why there are a "Beastmen race", temporary shape-shifting is a thing, but only changes the user's biology enough to allow for breeding of fertile children, which resulted in a mix of those who have vastly different appearances as a single race that's biologically compatible with humans. Those who just have minor animal features (ears/tail) had two human parents who got freaky with each other while using shapeshifting, while those who have more drastic animal features (upright animals with hands) actually turned into animals to breed with actual animals. What about the different race's magic, is it different because of their culture or biology? Is it that they are biologically superior in utilising magic in manner X over manner Y, that they can only utilise magic in manner X and are biologically incompatible with manner Y, or are magical manners X and Y magically incompatible, causing one to be limited to the one they're introduced to earlier regardless of which would be superior for the individual either personally, socially or biologically? Are the longer-lived races "superior" in magic, and, if so, is there an actual reason why they're stronger? It can just be as simple as "I am an Elf who lives three times as long as a Human, so I spent my first Human Lifespan mastering these spells to the level of a Human Archwizard before going exploring for my second Human Lifespan, and am now living the retired life of my final days in the Elf Village for my final Human Lifespan". I would also ask if you will incorporate any "immortality myths" between the sentient races that rarely see each other. For example, I know that Japanese Myth has a story where eating Mermaid Meat can make you immortal, in the un-aging sense, and some stories have "eating Elf Flesh gives you their immortality/lifespan" plot points (basically lies that cause some nobility to hunt elves for food). An amusing though I just had is that you could have a Goblin Race with a short lifespan (say about 2 decades before they die of old age) waging war against humans with an average lifespan (say 6 to 7 decades before they die of old age, due to lack of non-magical medical knowledge) to steal their "immortality", while humans are also waging war against elves with a long lifespan (say 15 decades, again limited due to lack of medical knowledge) to steal their "immortality". With the difference between the races being that Humans were the origin species, Goblins came into existence due to failed immortality experiments, and Elves came into existence due to a magic to create slaves that transformed their appearance and slowed the aging of the second generation of slave children. As for stories that have done varied lifespans well, that's difficult for me to answer. Even if a story does do it poorly or well, I may not notice it as lifespan being done well/poorly, instead I may just notice something I find interesting about the setting and ignore the overarching implication/s. Alternatively, it's brought to the forefront of the story for some reason (eg. hunting elves/mermaids for their agelessness, or elves becoming more sedentary as they age) but you either don't spend enough time with them to tell anything special, or they're just another race in the story and you can't really see the effects of their increased lifespan beyond the possibility of them being the MC's/Reader's "wifu fodder". Edit Add: When writing this I completely forgot about the manga "Sousou No Frieren", which has an elf living their life after helping defeat a demon king. I would recommend it if you wish to gain ideas on how to present a race that is almost immortal (Elves, like the MC), as well as another that's just long lived (Dwarves that pop up every now and again), because it is one of the foundational concepts of the story.
Ah yes, I meant an individual's lifespan! Thanks! YES I love Frieren! I forgot about that one too... And it's a mixture of races and species, (slightly arguably all species since the ones that can cross-breed are technically shapeshifters but I don't know how that would fall in the categorization since their original forms would not result in children at all so I probably should have put species in the thread) If it's at all relevant or helpful, think various prehistoric creatures that became sentient before humanity popped up and there wasn't a meteor to clean the metaphorical slate. Which brings to question number 2, yep, very different. Some are mammals-most are mammals, some are fish, some are bird-plant hybrids(don't ask), and so on. Each have their own specialties for magical/physical strengths and varying talents within each group, so they've got their own skill trees I guess you could say? Children are either one or the other, not a mix or half-breed. (so really all species I guess, in my defense my last zoology/biology class was in high school and it was botany) I had one bit about a mad scientist/witch/alchemist try to become a god and accidently create a plague, does that count? Otherwise I completely neglected the immortality mythos which is a huge oversight on my part, so thank you. The stairway approach to the immortality search is a really fun one and could lead to a lot interesting plots. ...and in the midst of my wild world building I may have forgotten the whole "relevant to the plot" bit...
Ah, plot, the one thing I try to come up with by worldbuliding enough that it spawns itself into existence, only to actively avoid the more interesting potential plots due to them being too dark for me to even consider writing, conscripting them only to the distant history of the world. Whelp, I mostly posed those questions to get you thinking about how to answer them, and use them as a basis to promote more questions for yourself. But, since you actually answered: So, a broad spectrum of races who are biologically incompatible, excusing shape-shifting shenanigans, which may go down to the genetic level. Who have different specialties in magic use, physical capabilities, and even preferred environments. And nobody's inherently immortal, there's just a difference, sometimes a significant difference, in base lifespans. Though you may be adding in a method for some or all of the races to become immortal, should they decide to train down that path at a young enough age. Honestly, what I would suggest is that you give a variety of lifespans based on the race, and have the short-lived races, with a base lifespan of 20 years or so, have a regular culture dedicated towards everyone becoming immortal, while the long-lived races, with a base lifespan of 2 centuries, have absolutely no desire to be immortal, particularly towards the end of their lives. Humans would be a medium-lived race, with a lifespan of about 70 years, by the way. This mostly would work if it is highly dangerous to just live in the world, to the point where it wouldn't be unusual for a grandparent to outlive their grandchildren due to the inherent dangers of daily life, regardless of which species it is. You can introduce the different species you created by having your MC's hometown be a midpoint for basically all likely environments, say a town with a coastline to the "west", a mountain range with snowy peaks large enough to form a desert on the other side to the "east", with a forest to the "north", and a plains to the "south". In fact, the only reason why the town is large enough to be a town is due to it being a good commercial hub for different species to meet, while simultaneously being too awkwardly far from any actually good marketable products each race wants that it can't actually grow to become a trade city, instead just being on the fringes of about 4 or 5 different cultural/racial environments in a spot that few of any authority actually ever care about.