Sometimes, when we see a race that is either immortal or very long-lived, it seems as if they age normally until approximately adolescence or early adulthood, and then stop completely. It is from this that we get the trope Immortality Begins at 20.
This trope is essentially the inversion, centering around a long-lived race with a proportionally long life cycle. A simple example would be to say that if this race lived twice as long as a human, then their childhood, adolescence, etc would also be about twice as long.
In the same vein, a race who lives half as long as a human will experience a much faster life cycle, experience a childhood that's about half as long as expected.
The trope applies primarily to a creature's natural lifespan and rate of aging. A character who gains slower or more rapid aging artificially or as part of something that occurs later in life is less likely to qualify for this trope. Artificially created creatures that have this lifespan and aging built into them or artificial procedures performed very early in life can be included.
Relative emotional maturity is also an expected element of this trope, but not a required one. Generally a character who is considered among his kind to be a child at age fifty will think and behave like a child. A character whose race reaches adulthood at age five will think and behave as an adult.
Note that in real life, most animals grow to maturity as fast as physically possible and then live a maximum lifespan roughly proportional to their likelihood of being eaten. I.e. bats mature in one year and live up to 20 because they fly, deer mature in 18 months and live about ten years, blue whales mature about as fast as humans and may live for centuries. That being said, it has been observed that the trope is generally Truth in Television, though the reality is that it's more of a case of correlating both maturation rate and maximum lifespan with reproductive quantity vs. quality rather than with each other; that is, the fewer the number of offpsring produced at a time, the slower they mature and the longer their lifespan, whereas rapid breeders tend to have both faster maturation rates and shorter lifespans. Outliers exist, of course, making the trope more of a helpful guideline than an absolute rule.
This trope will often overlap with We Are as Mayflies and may result in Mayfly December Romances. Compare Immortal Immaturity, when a character who is immortal acts immature and childish in contrast to their age; Age Without Youth, when a character has an extended or infinite lifespan but still grows old; and Not Growing Up Sucks, when a character is stuck in the childhood/adolescence stage and suffers in the process.
Longer Lifespan, Slow Aging
- Elves in Drifters are stated to have a lifespan about five or six times that of humans, and it shows in their rate of maturation. The youngest elves shown are young boys who, when asked, reveal their ages as thirty-six and thirty-nine. This shocks Toyohisa, since this means they're older than he is the Real Life Toyohisa Shimazu died in the Battle of Sekigahara being all of thirty years old (though in manga he often acts younger than his age would suggest).
- Miss Kobayashi's Dragon Maid: Kanna (and presumably all dragons), despite being Really 700 Years Old, looks and acts like a child because biologically and psychologically she is one, even if she's chronologically very old. This has significantly contributed to the "Don't Lewd the Dragon Loli" meme that has grown up around the anime adaptation of the comic.
- Downplayed in Soul Eater concerning witches' lifespans. In Soul Eater Not!, Kim turns out to be 16, not the 12-14 year old she was implied to have been mistaken for. Apparently, their development takes longer, but not as long as their lives are (for reference, Arachne and Medusa are over 800 and still look in the prime of their life).
- Part of the premise behind Maou No Kawaii Yamada Wa Yoiko. Yamada is a thirty-three year old salaryman who one day finds himself randomly in another world full of magical beings like demon lords, lamia, and dragons. However, all of these magical creatures have a far longer lifespan such that 33 is roughly their equivalent of first grade elementary school and, because humans are unknown and everyone comes in so many shapes and sizes, nobody thinks anything of treating the adult Yamada as a child.
- Hellboy: The eponymous character is said to age at one third the rate of a human being and thus has the physical prowess of someone at age twenty when he is sixty years old at the present time of the film.
- In the Honor Harrington series, Prolong has essentially tripled the human lifespan. The treatments must be done in early youth, and as the name implies prolongs every stage of human development. As a result, characters are Older Than They Look, with individuals in their twenties looking as potentially young as late preteens. The We Are as Mayflies overlap comes when interacting with planets or individuals who were too old to receive the prolong treatments when they became available. There's also one instance of a MayflyDecember Romance detailed in the Talbott Quandrant books.
- In the first book the titular character is over forty in Terran years, but being a third-generation Prolong recipient she appears to be in her early twenties and in some aspects displays the emotional maturity of a teenager.
- Hobbits in Tolkien's Legendarium live slightly longer than humans (to about 120, usually; keep in mind that at the time of writing, a human passing 90 was extremely rare), with proportionate aging. Merry and Pippin, in their thirties, are considered young adults, and Frodo, in his fifties, is just shy of middle age. Hobbits are also noted to have their adolescence in their "tweens."
- 120 is already an exceptional age for a hobbit, most of who easily pass the 90-mark, but rarely reach one hundred. It's not that the hobbits live much longer than humans, they just view age differently, considering life to really start at 40, when all the frivolities of youth are behind.
- In The Wheel of Time, Ogier can live for centuries. Loial, the one we see the most of, is ninety years old. By Ogier standards this is considered late adolescence. He is physically mature, but is considered among his kind to be emotionally immature and hot-headed. Nor is he considered old enough to leave the stedding (Ogier homeland) by himself.
- It's mentioned in Guards! Guards! that dwarf children aren't mature enough to be told about sexuality and reproduction until they hit puberty, at about age 50.
- Reaper Man has different life forms experiencing time differently depending on their life spans. Trees have a conversation that takes seventeen years and when one is chopped down, they seem to the others to have simply vanished.
- Ian McDonald's River of Gods and related short stories/novellas feature genetically engineered 'Brahmins' who live twice as long but age at half the rate, at least physically. They actually develop faster mentally, fast enough that spending two years as a helpless infant is a source of considerable frustration and misery.
- In The Lost Journals Of Ven Polypheme, Ven's race has a life cycle about twice as long as a human's, and they age proportionately. Ven is in his twenties but is still an adolescent. The explanation given is that they don't have to grow up as fast since they have longer to live.
- The exact natural lifespan of dragons in Spells, Swords, & Stealth is not stated, but it is assumed to be very long as a centuries old dragon who serves as antagonist of the third book is an "elder", but not "ancient" dragon, while a fifty year-old dragon the NPC protagonists encounter is still considered a hatchling.
- In one The History of the Galaxy novel, a Lost Colony turns out to be the site of an abandoned alien bio-lab. The colonists became exposed to a virus that mutated them into various forms: some gained invisibility and empathy (Shadows), others became shapeshifters (Metamorphs), yet others gained Psychic Powers (Emglans), and the fourth category didn't get any powers (Chosen). However, all four groups became The Ageless. Children age normally until maturity, at which point the aging process stops. Immortal Procreation Clause is in effect as well, as it's extremely rare for new children to be born. Despite the fact that, at about 20, the colonists are physically and mentally adults, the centuries of isolation have resulted in the colonists adopting this trope. Someone who is merely 150 years old is treated as a child. When a man from The Federation crash-lands on the planet, he is treated as an infant because he's in his 30s, despite the fact that he's a trained soldier and has more combat experience than all of the colonists put together.
- In Labyrinths of Echo the inhabitants of the titular city normally live to at least about 300, and can potentially live for thousands, depending on the personal magical power,note and they age accordingly, so Max, who is from our world, and is in his mid-thirties, was initially warned not to tell everyone his real age, lest he was tought of as a toddler. His friend Melifaro is his rough age-mate in all appearances, but is of ~130 years of age locally.
- Dragons in Birthright (2017) take about eighty years to physically mature. Mentally however, they seem about as mature for their age as humans, and are mature enough to leave their mothers at a mere six years old.
- Henry Kuttner's Hogbens. The narrator is a boy who looks and behaves like a teenager, and lost count of his age during Cromwell's times, while his younger brother is teething at the age of four hundred.
- Doctor Who:
- A Time Lord's rate of aging relative to their total lifespan is near impossible to determine, since when their bodies are burned out they rewrite their entire genetic codes and regenerate into a new body with new personality traits. Their aging within a single regeneration is clearer. We have seen the First, War, and Eleventh Doctors in old age, with either the statement or implication it took centuries to reach that point. The length of a Time Lord's childhood is unknown, but it's been offhandedly mentioned that Time Lords are still considered "kids" into their one hundreds.
- At one point, the 900-plus-year-old Tenth Doctor recalls one of his earliest adventures. "I came here when I was just a kid. Ninety years old..."
- In some fan multi-Doctor works, later Doctors consider the First and Second to be mere children, despite the fact that the First Doctor is physically older than them.
- In Magical Legend of the Leprechauns Mikey looks to be in his early twenties, but is stated to be "not even two hundred years old yet." We see leprechauns and fairies at all stages of life along with different people declaring themselves to be over a thousand years old.
- In the Blade TV series, pureblood vampires age extremely slowly. The most oft-seen pureblood Charlotte is about 200 years old but looks like a girl barely in her teens. Her outfit is decidedly Puritanical. However, her mindset is definitely not childlike, and everyone treats her with the kind of respect reserved for elders. The other purebloods also treat her as equals, aware of her actual age. Overlord Rusk, looking like a man in his late 30s or 40s, is stated to be over 600. It's unclear if they actually stop aging at some point. After all, turned vampires stop aging the moment they become undead.
- Dungeons & Dragons:
- D&D Dwarves for instance reach (physical) adulthood at 40 years old, are considered middle aged at 125 and can expect to live for more than three centuries baring violence. A D&D Elf is not considered an adult until he hits 110, meaning an ancient human, a Dwarf who is in the prime of his life and an Elf in the equivalent of her late teens might all have been born in the same year.
- Depending on the source, elves are often actually physically mature around the same time humans are (most of The Legend of Drizzt takes place when the title character is in his thirties or forties), but they're not regarded as adults by other elves until they've passed their first century. Some sources has it as a combination of social attitude (elves having a more modern attitude to when people are adults) and maturing at an increasingly proportional rate the more they mature (so elven babies stop being babies not long after human babies, while elven adolescence can last for decades).
- Warhammer 40,000:
- Space Marines age very, very slowly, since the process that turns them into superhumans begins around adolescence and takes years to complete. For example, these threeAstartesnote are 75, 175 and 250 years old respectively. However, it's usually harder to tell, since liberal use of Bald of Awesome hides the age difference.
- Juvenat treatments, reserved for the Imperial elite, drastically slow down the rate of aging, allowing some recipients to be pushing two centuries and still fighting (or running away).
- Implied with the Agori and Glatorian of BIONICLE, where in order for someone to be even remotely percieved as elderly, they apparently need to be well over 100,000 years old. Gresh is younger and acts a lot like a teenager, but even he is hinted in the story to be "only" a few dozen millenia old.
- This is also present to a lesser extent in the main story as well, since one character is subjected to a Rapid Aging attack, which advances him 3,000 years. He's treated as no worse for wear, likely having aged the human equivalent of less than a decade.
- The High-Entia are a winged elf-like race with a lifespan going into at least the 400s. Their physical ages are proportionate to their chronological ones. High Entia children have ages in the 30s-60s while young adulthood seems to be around the mid-late 100s and old age being the high 300s.
- The Machina are another example, having lifespans going well into the thousands and seemingly aging proportionate to them. However, being as they're machines/cyborgs it's unknown if they actually age as biological beings would.
- Fire Emblem Manaketes and other long lived races tend to age, physically and mentally at a slower rate then humans or their equivalent.
- In Mystery of the Emblem: Tiki is a 1000 years in human years, yet mentally and physically is equivalent to a ten year old human.
- In Fire Emblem: Binding Blade, the manakete Fa looks and acts like a young child even though she's several centuries old. It's mentioned in support conversations that manaketes just age that slowly—she will, eventually, look like an adult.
- Fire Emblem: Awakening, Tiki's expy, Nowi is 1000 years in human years, yet a child in dragon years, as a result she is mentally and physically a child. The english script tries to hide this as Nowi is marriageable to grown humans, but even there Nori's daughter, Nah says in the Harvest Scramble DLC that the mental age that Manaketes who look like children would have the mental capacity of a child.
- In Sword of the Stars, Liir reach maturity at the age of fifty and live for centuries, possibly millennia if they escape gravity's influence on the Square-Cube Law.
- BlazBlue: Celica has a special kind of soul that, along with bestowing her the ability of Healing Hands, lets her age more slowly and live for longer. Despite looking like someone in her mid-teens, she's actually a young adult. Her older self just looks like a woman in her 30s despite living for around a century.
- The Asari in Mass Effect can live to around a thousand years old and age slowly, seeming to reach the equivalent of sexual maturity in their 40s. Liara is 106 and seems to be the equivalent of her early 20s. They also have three distinct phases of life, Maiden, from puberty until around 350 when they enter the Matron stage and start having families, and then Matriarch around the age of 700 until their deaths.
- The Legend of Zelda:
- Downplayed and played with concerning the Zora in The Legend of Zelda: Breath of the Wild. They live to be a bit over 200, and the Zora princess Mipha notes at one point that Link, a Hylian presumably around the same age as her (17), looked grown up faster than her (he's One Head Taller than her despite adult Zora being One Head Taller than the average Hylian). There's also Finley, a Zora girl who appears to be a short child but complains about not yet reaching that final growth spurt and also speaks in a very formal and adult diction, indicating that she's in her mid-teens at the youngest; she eventually hooks up with a Hylian guy who appears to be in his 20s at the oldest, and her mother doesn't see anything wrong with this. This is in contrast to Zora princess Ruto from The Legend of Zelda: Ocarina of Time, who seems to grow up at the same rate as Link.
- The Champion's Ballad DLC confirms that Sidon, Mipha's younger brother, was born during Mipha's lifetime, despite her having been dead for a century in the present. Since Sidon is a child in his flashback with her, and a not-particularly-old-looking adult in the present, the implication is it took him most of the century to fully mature.
- Dragons in Final Fantasy XIV are magical in nature and can become The Ageless, but it takes a great deal of time for them to mature to such a point. Dragon whelplings are physically and mentally children for hundreds of years.
- Elves in The Order of the Stick are shown to experience this. Vaarsuvius' two children are 26 years old. For elves, this is the equivalent of kindergarten-age. In the prequel On the Origins of PCs, V mentions spending 20 years in diapers as a downside to slower aging.
- Kayoss in L's Empire age about 100 times slower than humans. Although they grow proportionally to that of a human, they don't reach sexual maturity until they're in their 2000's.
- Scientifically Accurate Ninja Turtles notes that since turtles live for two centuries, "teenagers" would have the minds of seven-year-olds.
- Wonder Woman in DC Super Hero Girls is 317 years old, but her mother isn't just being coddling when she refers to her as an adolescent—that's why she's so much shorter than the adult Amazons.
- While the total lifespan of the titular Gargoyles is not stated, in the Season 2 episode "Grief" Goliath explains that Gargoyles age at half the rate of humans because they don't age while turned into statues during the day. This can be seen when Goliath and Eliza meet Tom. Tom was a child back in the Manhattan Clan's original time and helped care for the unhatched Gargoyle eggs in the castle rookery. When they meet Tom as an adult, he's old enough to have significant amounts of gray in his hair, but the Gargoyles he helped nurture are still adolescents.
- Steven Universe:
- Steven may have something like this going on—he's the first gem/human hybrid, so no one's sure how he ages. Eventually, it's shown Steven aged normally for eight years, seemingly stopped for six, but then suddenly showed signs of reaching puberty (a single facial hair). However, another episode suggests he may change his age unconsciously or even at will, thus he might be able to overcome this disadvantage.
- Word of God has confirmed that Lion and Lars are not outright immortal, but rather will age very slowly compared to other members of their respective original species. This explains Lars' very slow heartbeat felt by Steven after he died and was revived on Homeworld. At least one of them was almost fully grown by the time they Came Back Strong, negating most of the downsides of this.
- In the special episodes of Wakfu, Yugo discovers his Dragon Ancestry has made him Long-Lived, but is displeased that he still looks ten as he's reaching his twenties.
- The Greenland sharks don't reach sexual maturity until they are 150 years old and some individuals seemingly can live to be at least 400.
Shorter Lifespan, Fast Aging
- Windermerans in Macross Delta are a subspecies of humanity created by the Protoculture to have incredible physical abilities at the cost of a radically shortened lifespan. The average Windermeran lives for barely 30-35 years. It's played with in that it mainly concerns their social age, rather than biological. Frejya, for example, is fourteen, and looks it, but is considered an old maid and enters the story to escape an Arranged Marriage with a potential suitor who, she believes, she is Better as Friends with. Helman is already an old man by their standards at 33, with confirmed grandchildren, but he doesn't look any older than that.
- A Christmas Carol: As Scrooge and the Ghost of Christmas Present go about watching people celebrate the holiday, Scrooge is surprised to notice that the Ghost is rapidly aging. The Ghost explains that his lifespan is only a single day and ends that night.
- In the Star Wars prequels, clone troopers age twice as fast as baseline humans. Expanded universe materials mention they also live half as long, and an attempt to fix this is used as a subplot in the Republic Commando Series.
- Reaper Man has different life forms experiencing time differently depending on their life spans. Elderly mayflies at sunset are seen reminiscing about how the sun was properly yellow and high in the sky when they were young.
- Star Trek: Voyager: The Ocampa only live 9 years, and by age 1 are fully adult.
- In Dungeons & Dragons half-orcs reach adulthood at 14 and are considered venerable at 60 with a max age of 80, humans are adults at 15 and venerable at 70 with a max lifespan of 110. Full orcs presumably have it even worse.
- In Shadowrun all metahumans are legally considered adults at 18, but orks are physically mature at 12 and live 35-40 years.
- In Sword of the Stars, Zuul grow to full size in a matter of months and usually die of natural causes before they turn 40.
- The hozen of World of Warcraft are a very short lived race, having lifespans of about twenty years. Their elders are typically around thirteen or fourteen years old.
- Winston, Overwatch's resident uplifted gorilla is 29 years old but comes across as middle aged. Real life gorillas live about 50 years.
- Kevin & Kell has played this for laughs with actual mayflies. Such comics often show them being born and dying within the course of the same strip.
- In Public Humiliation pookas mature in about a year, their lifespan is never given specific numbers but it was short enough for Boo to break up with her longer-lived boyfriend for fear of leaving him when he still had his life ahead of him. However, it's discovered that semigods like Lan's progeny, including Boo, have the proportional equivalent to Immortality Begins at 20, and they can pass that longevity onto their mates.
- In Fantastic Mr. Fox, the concept of fox-years implies that foxes age six times as fast as humans. Ash is two years or twelve fox-years old, and is about as mature as a twelve-year-old child, while Mr. Fox is seven years old (which would be forty-two fox-years), and has midlife crisis.
- The Petalars in ThunderCats (2011) age from childhood to death by old age in a single day. From their perspective, it feels like a long life.
- The traditional illustrations of Baby New Year, who is a baby on January 1st and ages into an old man by December 31st, when he dies to be replaced by a new Baby New Year.
- Some other great apes have life cycles proportionally similar to humans, but shorter lifespans.
- Gorillas often reach maturity at 9-12 and typically live to 40 in the wild, 50 in captivity, with 60 as the record.
- Chimpanzees are mature at about 10 and have reached their 60s in captivity, with at least one specimen who lived to 78.
- Orangutans are sexually mature at 15, there's little data on their potential longevity but one has been recorded living to 55.