In fictional works regarding either races that are unusually long- or short-lived, one pattern continually crops up — regardless of a character's real age, they will always have an emotional maturity matching their physical maturity. A one-hundred-year-old elf who looks like they're in their thirties will always act like they're in their thirties. A child who has grown to physical maturity in a few weeks will be exactly as intelligent and oftentimes more so as their physical appearance would imply.
Where this trope can get really funky is when an Interspecies Romance is involved, particularly the MayflyDecember Romance variety. While people in Real Life generally cringe at the idea of a man in his 70s romancing women in their early 20s, this becomes immediately palatable in fiction as long as they physically appear to be the same age. Whether this says more about the motivation of Squick in Real Life or fantasy writing in general is hard to say.
Naturally, this can be explained, to an extent, with Bizarre Alien Biology and differing cultural customs. Not having known of any other intelligent life forms, we can't rightly say for certain how or if emotional maturity would progress with creatures that have a different lifespan than we do. In many cases, though, it still requires that younger members be an Instant Expert in order to learn as quickly as they do.
Very common in stories featuring intelligent robots, where robots only a few years of age will have grasped everything important about human society and interactions. They tend to get a pass, of course, for not really looking a specific age (well, most of the time).
See Immortal Immaturity for the sub-trope describing this phenomenon specifically as it relates to the attitude and emotional quotient of exceptionally long-lived characters. This trope is more focused on the comparative interpersonal interactions of multi-generational casts.
Contrast Not Growing Up Sucks, which inverts this by showing that a 50-year-old in a 10-year-old's body will, in fact, act like a 50-year-old. Compare Age Is Relative. Compare Acting Your Intellectual Age, where intellectual maturity automatically equals emotional maturity.
- Sasha in Uchuu Senkan Yamato is adult at around 1 year old, with full language skills.
- Mahou Sensei Negima! zig zags this one with Evangeline, who constantly flip-flops between a "seen-it-all" mentality (she's Really 700 Years Old) and the emotional maturity of a child (because The Mind Is a Plaything of the Body).
- Averted in Elfen Lied, where Silpelit diclonious characters grow to physical and intellectual maturity in half the time of a normal human, but develop emotionally at a normal rate. For example Nana, who is chronologically 6 years old, has the physical maturity of a 12 year old but behaves as if she is six.
- Haku in Spirited Away is a centuries old river spirit/dragon. Hints of romance between him and the 10 year old protagonist are made to seem okay because he looks like he's the same age.
- In Howl's Moving Castle Sophie starts as an old soul in a young girls body, but after she gets cursed her age starts to fluctuate based on her mood and enthusiasm for a developing relationship with a young man.
- Wolfram from Kyo Kara Maoh! is an 82-year-old demon who looks maybe 16 and acts like an even younger bratty teenager.
- In Sailor Moon, Chibi-Usa is Really 700 Years Old and looks around seven. At one point, she says she's "not a child" (when she sees an ice cream shop), but she consistently acts like a seven-year-old.
- Happened with Gwen Stacy's children in the Spider-Man story Sins Past, who grew up fast by Applied Phlebotinum because Comic-Book Time means they would only have been 9 years or so old. Imagine this cover with a 9 year old.
- There was a clumsy attempt to invoke this trope when Hal Jordan hooked up with the teenage Arisia; Arisia unconsciously used her ring to make her body become mature, and claimed that her race matured fast emotionally, which would make the relationship okay by this trope. Readers who still remembered that Arisia recently looked and acted teenage couldn't accept this, which led to Arisia's actual age being retconned an number of times. It ended with her age being 240 in Earth years, so it is now her teenage form and not her adult form that fits the trope. The current status of Arisia's age in the New 52 is unknown.
- In the shockingly good Peggy Sue Harry Potter fanfic, Always and Always, this is discussed by Harry and Ginny. They've both gone back in time to fix things, and they're trying to figure out what is an appropriate age to resume having sex. They ultimately decide that most of the dangers of teen sex (with proper protection, etc, naturally) are emotional rather than physical, and settle on fifteen years old. Agree or disagree, at least they addressed the issue.
- Inverted in the brief but chilling Twilight fanfic by Estora, Seven. In the story, which is told by Renesmee Cullen, she looks eighteen, but she's mentally and emotionally seven...something that her vampire family and her werewolf imprinter choose to ignore.
- Basically, The Island significantly justifies and mitigates the Fridge Horror Squick of a boy and girl who are chronologically only three and four years old, respectively, having sex by applying this trope. Technically, though their memories of childhood and adolescence are all fake, and the ones who cloned them deliberately kept them in the dark about sex and sexuality, those fake memories do provide them with enough simulated years of experience that they can plausibly be as emotionally mature as they should be at the age they appear to be (i.e. twenty-somethings).
- This trope is referenced early in Down and Out in the Magic Kingdom: The hero's girlfriend is 23 years old, roughly a quarter of the time he's been alive. For the duration of the book, he has a body of about the same rough physical age. This is lampshaded, as he acknowledges that earlier in his life, this fact probably would have bothered him.
- In The Mortal Instruments, this is true up to a point with warlocks. Most of them stop visibly aging when they reach adulthood, and they do not become elderly people in young bodies usually. Magnus in particular somewhat consciously tries to act the age he looks. But he also states that older warlocks do start to become emotionally detached. Fairies and vampires are more variable though. For example, Raphael Santiago looks about 14 or 15, but acts much older, which he is. Aside from Fantastic Racism, most people tend to interact as peers despite real age differences. Arrogant Clave members think nothing of acting older and wiser than Downworlders that are many times their age. A romantic relationship between Alec (18) and Magnus (400+), is not seem as problematic except for the MayflyDecember Romance issue and Alec's later jealousy over the sheer number of past lovers Magnus has had.
- Twilight is a painful case. Ignoring Edward and Bella (which if you accept the idea that physical and mental age staying the same can make a lick of sense, they mention that two year old vampires are forever two, is still mildly cringe worthy), you also have the last book couple. A 22 year old man and a 7 year old girl, and it's okay with her parents.
- Played with in Thief of Time, in which the abbot of the History Monks was recently reincarnated as a baby boy. While he can usually speak with a wise old man's intellect, his dialogue keeps being interrupted by his toddler-body's overriding, infantile demands for Bikkit!
- Examined through The Mind Is a Plaything of the Body in The Master Mind of Mars. When Ras Thavas' brain is transplanted into the body of a younger man, he has the mind of the centuries-old scientist that he is, but the hormones of a strong, virile and wrathful man in the prime of his life. He tries to maintain his more "mature" (emotionally-detached) outlook, but it's a losing battle.
- How well this trope would actually apply in the event of a "Freaky Friday" Flip between parents and their children is thoroughly examined in We Can't Rewind, leading to some rather unsettling conclusions: while it doesn't reduce the adults' emotional maturity or improve the children's, it does awaken very mature sexual desires in the children's minds—leading to some very mature sexual activities, much to their parents' distress.
- Subverted in David Eddings's The Elenium series. Aphrael, known as "the child goddess" appears, and mostly acts, like a six-year-old child. It's all a ruse to get people, including the other gods, to underestimate her. In fact, she's actually shown to be significantly more mature than most of the other gods we meet.
- On Bewitched, Samantha is hundreds of years old, yet she acts like the young woman she appears to be. Especially in early seasons where she is more demure and less assertive than in later ones. Her cousin Serena is an even more extreme example, as she is roughly the same age as Samantha but behaves very much like a teenage girl.
- Appears in Star Trek: Voyager with the Ocampa, who are unusually short-lived but possess cognitive faculties greatly in excess of what you would expect from a race where nearly all the members are less than 10 years old. This gets exceptionally extreme when Harry Kim marries Kes' daughter, who's only 2 years old.
- It also shows up with the Doctor, a computer program with no sense of personal attitude or interaction at all who, for most of the show's run, is emotionally indistinguishable from a normal man in his mid-40s. His personality is largely based on his creator, who himself is not what you'd call personable. The Doctor mostly grew out of this through his interactions with the crew.
- Icheb, a former Borg drone, was only recently assimilated and aged from childhood to his late-teens in a maturation chamber. While sometimes socially-awkward, his maturity nonetheless matches his physical age, and even Captain Janeway disputes the assertion that he is "just a child".
- Shows up in Star Trek: Deep Space Nine with the Jem'Hadar, who rapidly age into physical maturity and, in the same time frame, master the higher levels of intelligent thought.
- Not to mention Jadzia and Ezri Dax, who hardly ever demonstrate their centuries worth of wisdom and experience.
- Odo has the appearance and demeanor of a middle-aged man. However, he was adrift in space as an infant Changeling for an unknown amount of time, so he is likely much older than he appears.
- Averted for laughs in 3rd Rock from the Sun, where the aliens' body assignments were done independently of their real age.note As a result, even though Tommy is the teenager, his alien mind is easily the oldest of the group. Consequently, Tommy is the most emotionally capable member of the crew, and it often falls to him to give the others advice when they've dug themselves into a deep hole. This leads to some hilarious visuals, like the time Dick (officially Tommy's father) was sobbing like a child and Tommy had Dick sit in his lap while he consoled him.
- Of course, when Tommy himself gets in trouble misunderstanding human culture, he often gets the worst of it since he has no one available for help.
- Liam Kincaid in Earth: Final Conflict is born in the second season and grows to adulthood within one episode, at which point he's written as and treated as fully adult.
- In Wizards of Waverly Place, Juliet is a 2000 year old vampire who is the girlfriend of the 17 year old Justin. However, she looks and acts like a teenager, and even interacts with her parents like one, making it seem okay.
- In the first episodes of Buffy the Vampire Slayer season 4 Buffy's demonic roommate looked (supposedly) and acted like a teenager. When her guardian appeared to take her back to hell she complained that she was 3000 years old, but he still treated her like she was 900. Averted with Colin, who was actually more reserved than The Master.
- Doctor Who:
- Played with. Both age and maturity level vary with each regeneration, but the "Father Chirstmas" element of an old man who delights in childish things never entirely goes away.
- In multi-doctor crossovers, the one who is physically oldest will generally take charge, even though the older actor is usually playing a younger character.
- In The Three Doctors and The Five Doctors, the one who acts with greatest authority is the First Doctor.
- In The Two Doctors the Second Doctor takes charge over the Sixth.
- In The 50th Anniversary, Ten and Eleven get along like old school chums, but The War Doctor seems to be the internally conflicted authority figure.
- When Eleven regenerates into Twelve, he comes out as a man significantly older than most of his more recent regenerations, and acts accordingly. Madame Vastra theorizes that he subconsciously did this to remind both himself and others that he is much older than he had been acting recently (Eleven, in particular, was often described as "an immortal god who insists on acting like a ten year-old").
- Just how acceptable it is for there to be any romantic implications between the Doctor and his companions is relative to the Doctor's apparent age. This can get complicated, as a few of the Doctor's later incarnations are physically younger looking than some of his earlier ones, despite being chronologically older. In particular, cases such as the relationship between Ten and Rose would have been considered Squick if you substituted One, Two, Three or Seven. Even her relationship with Nine had more of a mentor/student feel. Also, the Doctor himself tends to experience personality shifts that adjust to his current incarnation's apparent age. Notably, the War Doctor, dresses down Ten and Eleven as if they were immature naughty boys, despite the fact that they are both older than he is. One is always treated as if he were the eldest Doctor, even though he is in fact the youngest.
- Malia Tate on Teen Wolf. A werecoyote, she transformed for the first time when she was a little girl and apparently murdered her mother and older sister. She then spent the next eight years living in the woods as a coyote due to the emotional trauma. When Scott uses his Alpha powers to change her back into human form, she is now a teenager. Physically in her late-teens, she is automatically treated as part of her chronological age group by the writers, including it being seen as okay for her to have sex with Stiles. This despite the fact that she had no education or human socialization during the eight years she was a coyote.
- The Stargate SG-1 episode "Brief Candle" revolves around a planet where people only live for 100 days, and children appear to age a year each day. Our heroes are disturbed by this rapid aging, but nobody seems to object to Thetys having a baby when she is chronologically 20 days old, and everybody on Argos who has reached physical maturity is treated as a full adult.
- Harukanaru Toki no Naka de:
- The manga has an interesting subversion. Abe no Yasuaki is an Artificial Human created about two years prior to the start of the story, who looks like an adult (his "official" age is 21), for the most part acts like one, and possesses sufficient amount of knowledge. The catch? He is initially devoid of emotions, and his understanding of anything related to these tends to be on the level of his real age. Akane actually compares him to a small child on several occasions before learning of his origins.
- Both played straight and subverted with Human Hakuryuu in Harukanaru Toki no Naka de 3, an ancient deity in the form of a ten-year-old boy whose behaviour is more or less appropriate to his physical appearance; it is implied that he simply has no experience with living among humans — hence the childlike innocence which he retains even after achieving a more adult form.
- Adult Tarutaru from Final Fantasy XI look very young to the human eye and are very commonly characterized as immature. Particularly striking with the Chebukki siblings in Chains of Promathia; the cast includes every race and nationality seen up to that point, and the Tarutaru representatives are nominal young adults... who are a bunch of bumbling, mischievous brats whose subplot is obsession with finding their parents.
- Their racial Expy in Final Fantasy XIV subvert it, however, by acting very much like the adults they actually are.
- In Disgaea, do 1313 year old demons act and look like they are 13? Yes, yes they do.
- Tends to be the case for the characters who are dragons in human form or part dragon in the Fire Emblem series, despite being as much 100 times older then they appear to be.
- Played oddly in Tales of the Abyss. Luke is a clone, physically in late teens but in actuality only seven, with childlike emotional maturity to match the actual age. All the other characters seem to expect this trope to be in play, and react as if the character in question was evil or an idiot for not fitting their expectations grounded in this trope.
- Used in the backstory to Warcraft III: Arthas and Kael'thas were rivals for Jaina, but as Jaina and Arthas were young humans and Kael'thas a young-looking Really 700 Years Old elf, she was considerably squicked by his attentions.
- This trope is inverted in Touhou; ZUN has explained that the 500 year old loli vampires Remilia and Flandre Scarlet are eternally young because they have forfeited their growth and maturity in exchange for eternal life. In other words, they can live forever because they never grow up.
- Averted with Finley in The Legend of Zelda: Breath of the Wild. While she looks like a short young Zora girl, she's actually a young adult who has yet to go through her final growth spurt (likely a few decades old if Zora growth rates and lifespans are anything to go by) with the mental and emotional maturity her real age entails; in fact, she acts more formal and mature than her own mother, Kodah. She ends up falling in love with a Hylian man named Sasan who is in his 20s at the oldest, and while Kodah doesn't seem too enthusiastic about her daughter's choice of man, her objections don't seem to be based on their differences in physical or actual age.
- In Beyond Bloom, Sikue and Tatsuma appear as teenagers, but are said to have not been alive for very long (at least a year). Despite this, they are fluent in speaking. This is somewhat subverted when the girls are shown to act very naive and childlike, and have a lack of awareness of what is socially acceptable.
- Jenny from My Life as a Teenage Robot was designed to act like and have the emotional/mental capacity of a teenager, despite having been built only five years ago. One episode even sees her being forced to go to kindergarten because she is legally a toddler; her mother eventually pulls some Loophole Abuse to get her out, demonstrating that while Dr. Wakeman completed building Jenny five years ago, it took much longer to plan her out schematically and put together all her parts.
- The original ThunderCats (1985) series played this mostly straight: Lion-O was, chronologically, about the same age as Wilykit and Wilykat, but matured to adult size (while in cold sleep) during the trip to Third Earth. He generally acted like a grown-up, but on the occasions he acted his true age (like a kid), he'd generally get scolded for his "immaturity". Justified in that, as "Lord of The Thundercats," Lion-O had a standard of behavior to uphold.
- Young Justice has Superboy and M'gann who are respectively 1 and 48, yet both are considered in the 16-18 range thanks to artificial growth and Martians aging three times slower than humans. In a subversion, Conner has a lot of emotional issues, and at times can barely suppress his rage.
- Steven Universe:
- Gems are born fully grown then never age, but their appearance of age tends to reflect their personality. Amethyst, Peridot, and the Rubies are all short, young-looking, and very immature. Yellow Diamond, who's been a leader of Homeworld gems for many millennia, has some pretty heavy lines on her face. Flashbacks to when Greg met Rose (very recently in terms of a gem's usual lifespan) showed Garnet, Pearl, and especially Amethyst as considerably younger-looking and less mature than in the present.
- In "So Many Birthdays", Steven changes to whatever physical age he feels like. Steven has a Maturity Is Serious Business moment and suddenly ages, until it escalates and he becomes an elderly man on the verge of death. Fortunately, he eventually remembers how to have fun and return to his kid form. His appearance being linked to his emotional maturity could explain why, as a Kiddie Kid, he's fourteen and hasn't changed in physical appearance since he was eight.