Typically, a main or recurring character who regularly interacts with characters noticeably younger, usually resulting in their maturity level regressing to roughly the same level (if not even lower). They're still capable of acting their age... they just usually don't.
This character shows up most often in anything aimed at younger audiences, where they are Played Straight so as to be more appealing to kids. When Played for Laughs, it's to show an immature or silly character.
Contrast with Wise Beyond Their Years, as One of the Kids is rarely treated as any kind of authority, because of their casualness they often do not even earn special respect.
- Yukari Tanizaki from Azumanga Daioh could be considered to embody this trope, although she is still treated by her students with respect due to being a teacher (by everyone except Tomo). A respect she abuses without shame.
- Adults in Lucky Star are often this, to the annoyance of Konata.
- Her homeroom teacher Nanako Kuroi is envious about students needing to wear uniforms, as she found choosing clothes a chore. And one time, she decided the game when her favorite baseball team lost did not exist at all. Due to being single, she also spends a lot of her free time on MMORPGs, where she frequently interacts with her student Konata; though unlike the latter, Nanako is wise enough not to sacrifice working time for playing time.
- Her cousin Yui Narumi is a traffic cop who Drives Like Crazy, is very cheerful and energetic. Despite being a young woman, she can also be immature, especially compared to her teenage sister and cousin, who she often visits.
- Ranma ½ gives us Hinako Ninomiya, the English teacher that is in her late twenties, looks around twelve, and behaves like a six/seven year old would. Although she only acts in her child form. In her adult form she's usually all business, unless she stays in it for too long.
- Yotsuba&! has Yanda, who during his first meeting with Yotsuba proves that he's just as immature as she is — and often goes too far in making himself Yotsuba's Sitcom Archnemesis by stealing her food and calling her names — and big guy Jumbo, who starts a one-sided competition with ten-year-old Miura over visiting Hawaii. Yotsuba's father, while more responsible than the other two (being Yotsuba's legal guardian and all), is not above playing along with her antics. However, the three are perfectly capable of speaking calmly like normal adults when the kids are not around.
- Kasugano-sensei from Sketchbook acts very childish at times, which also gets commented on by the students in her art club.
- Yoshinoya-sensei in Hidamari Sketch is a very good example. She considers herself a young woman before even thinking of herself as a teacher, delights in dressing up in costumes, and is prone to odd looks from her students. Her principal is always extremely disgusted with her, and Hilarity Ensues whenever they meet.
- Kaneru from Doujin Work is an office lady with a very childlike disposition, which makes her bond with Token Mini-Moe Sora very well.
- Jack Rakan in Mahou Sensei Negima! doesn't count himself among the main cast, but he's managed to become a major player in the kid/teen-filled Ala Alba. Keep in mind, he's old enough to be considered a Bad Ass Grandpa. The other Ala Rubra members (Eishun, Albireo) by contrast had a sense of aloofness to the main cast.
- While Daikichi Kawachi has this as an Informed Flaw in Bunny Drop, he quickly grows out of it raising Rin. Rin's mother Masako on the other hand, is a much clearer example, with Rin's biological father (and Dankichi's grandpa) stating in a will that Masako wasn't yet mature enough to be a mother, and Daikichi's impression being that Masako simply doesn't realize the problems she's leaving to other people by not taking Rin.
- Soul Eater:
- Michiko Malandro from Michiko to Hatchin sometimes shows signs of being less mature than her travelling companion/surrogate daughter, Hatchin.
- In A Certain Scientific Railgun, Mikoto Misaka is known among people who are close to her for having some really childish tastes (such as an obsession with a frog mascot marketed at three-year olds), not to mention her childish inability to accept defeat when fighting Touma. Kids notice this, which is one of the reason she works well with them.
- Sein of Lyrical Nanoha. She could act her age if she needs to, it's just that she usually doesn't. She even does an all-out temper tantrum when she gets caught red-handed doing one of her pranks in ViVid.
Nove: Geez... it makes me feel depressed when I realize that you're supposed to be older than myself.
- Read or Die:
- Yomiko Readman lapses into this quite a bit, when she's not killing people. In the manga, she once expresses her displeasure by making a paper airplane with her making a rude face and throwing it at her opponent.
- And let us not forget the second Nancy clone from the OVA/TV series, who has the mind of a five year old due to brain damage and amnesia.
- Dragon Crisis!: Eriko is unable to cook and irresponsible, leaving her younger cousin Ryuuji to be the grown-up one.
- Kotetsu T. Kaburagi aka Wild Tiger of Tiger & Bunny might as well be the embodiment of this trope. He is a thirty-something father and veteran superhero. This does not stop him arguing with small children over trading cards, acting like a ten-year-old fanboy at the mention of his favorite superhero (Mr. Legend), riding animatronic rides meant for little children at the mall while waiting for his partner/talking on the phone with his daughter, or replacing the background of his coworkers' cellphones with pictures of himself making silly faces.
- Irabu-sensei from Trapeze is this. His Adult and especially Stuffed Animal personas act childish, silly and borderline crazy awesome (for normal and average people around him), but when he switches over to his Child persona, he acts serious and like a decent psychiatrist.
- Handa, the 23-year-old main character of Barakamon, is one of these, often hanging out with seven-year-old Naru and her friends.
- Kei's mom in Houou Gakuen Misoragumi. She's also considerably ignorant to go along with her immaturity.
- Kagura from Gintama is often showing hanging around the neighbourhood children and playing games with them. Arguably Sougo too since he acts very childishly sometimes, especially when he's around Kagura which can cause them to descend into playground-scale wars.
- In One Piece 19-year-old Luffy and 8-year-old Momonosuke get into several funny and heated arguments because Luffy is just as immature as Momonosuke and probably even dumber.
- While most Pokemon in the Pokémon anime tend to gain a more mature and aloof personality after evolving, there have been exceptions.:
- Compared to the rest of Ash's Kalos mon, Goodra remained as Adorkable and playful as it was as a Goomy, which lends to several characters recognising it as such just from behaviour. When it reunites with it's childhood friend, Wooper, it goes into an excitable fit, despite now being over ten times it's size.
- Ash's Lycanroc in the Sun and Moon series tends to act as much like friendly and energetic puppy as it's Rockruff form and even has a Friendly Rivalry of sorts with the unenvolved Litten.
- Kiawe's team consists of the enormous powerhouse Turtonator and the evolved Alolan-forme Marowak. Both engage in the class' fun and games as much as the cuter Pokemon and are often seen bickering or scrapping like childish siblings.
- Ilulu of Miss Kobayashi's Dragon Maid is stated to be the same age as Tohru, roughly the dragon equivalent of a sixteen year old. However, due to past childhood traumas, she's emotionally stunted and she's shown to vastly prefer the company of younger children such as Kanna and her elementary school friends. She can be mature, but even then her preference for the company of kids comes through. Her choice of job, part-timing at a candy store, allows her to contribute to the household, as Tohru insisted she do, but also means she can work in a place children often visit.
Ilulu: Kids... you know... I like them... I like playing with kids... but when you have a job you can't play... so at least I want to be in a place where kids look like they're having fun... is what I thought.
- Grandpa from The Beano acts like a child and plays with children but as his name suggests is elderly (although he is never shown to have Grandchildren) and also has an even older dad who used to beat him with a slipper if he misbehaved.
- Buddy the Elf in the movie Elf, in which most of his actions are naive, adult child-related, not elf-like. In fact, he's just about an opposite to all of the other elves in the film.
- Maria from The Sound of Music loves singing and dancing and being a free spirit. This concerned the nuns at the Abbey, as she at first wanted to be a nun.
- Brand in The Goonies. He spends the first act of the film trying to keep his little brother and his friends in line, but he eventually gets swept up in the adventure as much as the rest of them.
- Mac and Kelly in Neighbors. Leads to a pretty humorous argument where he feels Kelly should be the responsible one.
- X-Men Film Series:
- X-Men: First Class: Raven would rather hang out with the adolescent mutants than with Charles and Erik even though her age is much closer to the two men's.
- In Diary of a Wimpy Kid, almost all of Rowley's friends are little kids.
- Gordy the janitor on Ned's Declassified School Survival Guide who helps with many of the main characters' simple plans.
- Spencer from iCarly. Worse, he's a Promoted To Parent older brother to the main character. Yeah...the only parental figure Carly has is more of a kid than she is. At least she's a high schooler, starting with season 2.
- Despite being an over-1100 years old Time Lord, the Doctor in Doctor Who demonstrates a few of these trait, in particular the Eleventh Doctor who is a Manchild and very close to children.
The Doctor: There's no point in being grown up if you can't be childish sometimes.
- As befitting the Doctor's foil, the Master (especially when played by John Simm) is something of a Psychopathic Manchild.
- The Doctor is the man who responded to Amy Pond saying "I grew up" with "Don't worry, I'll soon fix that" and who uttered the following quote back in his Fourth Doctor days:
- The Twelfth Doctor begins showing some of these traits in his second season travelling with Clara Oswald, dressing more casually and gaining an interest in rock and roll and electric guitars. In one episode he also openly addresses Clara as he would a teacher.
- Pee Wee Herman, obviously.
- Cliff Huxtable of The Cosby Show may be a doctor, but he often has childlike traits. Mostly he uses them only around children.
- Gibson in A.N.T. Farm acts in this way most of the time. He seems to have a mental age below that of the kids - even assuming the ANTs were regular highschoolers, let alone of an age when they'd normally be at middle school.
- Dr. Rodney Mckay from Stargate Atlantis is one, so much so that in one episode a little girl brattier than him calls him out on it.
- Fran Fine from The Nanny, which is one reason she's such a great nanny to the Sheffield children.
- Winnie Goodwin from the short-lived ABC show Free Spirit. In addition to providing the page quote, Platypus Comix has this to say about her:
Winnie is centuries old, but in terms of development by human standards, she's in her early twenties — in looks and in attitude. Not only does she understand the Harper children in a way most grown-ups fail to, she often has the same wants and desires.
- Pearls Before Swine:
- Pig is eternally clueless and innocent compared to the world around him. All of his friends, particularly his roommate Rat, ridicule him for this.
- Rat himself can act very childish often, though in a more selfish, bratty way than Pig. An example of this is him separating himself and Pig into a "Cool Fence" and "Uncool Fence". Also, he prefers for people to give him what he wants, despite Goat and Zebra's urging to make him think otherwise.
- Wooten Basset from Adventures in Odyssey skirts on this trope. He has many child like interests and behaviors but he also retains a perceptive intellect. It is implied that he acts the way he does out of choice, as his parents raised him to be as strict and disciplinary as possible, not allowed to experience the joys of childhood like playing outside or even having toys.
- Ray and Ed from the Peacock And Gamble Podcast seem to both have this as their default setting. Ray, in particular, is obsessed with Star Wars lego, and Ed frequently calls him out on the number of bizarre children's toys and memorabilia he has in his house - including a Gamorrean Guard's head, and a full-sized working replica of Gonzo from the Muppets. They both like to speak in a sort of blissful, giddy idiom, too.
- All the human characters of Ms. Bleep have regressed to this, having spent an unknown amount of time "trapped in school in perpetuity" with a robotic kindergarten teacher that gives them an electric shock every time they leave or misbehave. Cindy, the resident brown-noser, doesn't even seem to remember the outside world or the fact that she's an adult, and has no way of comprehending the death of a fellow captive. The other two are trying to escape, but they're still jarringly childlike in regard to such subjects as cookies and naptime.
- Mary in Vanities, mainly due to her difficult childhood. She finally matures in the finale of The Musical.
- Weston Hurley in Lanford Wilson's The Fifth of July could certainly qualify.
- Felicia the Catwoman from Capcom's Darkstalkers saga has been slowly transformed into one of these, as newer games shows her throwing kiddy tantrums, being dense as a brick, losing her head for a piece of food and hanging around with kids like Klonoa (in Namco × Capcom), behaving exactly like him. The weird part? Even if she is genuinely friendly and never holds any ill intention, you must remember that she's 41 years old. The original description says catgirls age by about half the speed of a human. Technically, she's 14.
- The "Child at Heart" perk in Fallout 3 gives assorted bonuses to interactions with child characters.
- Alistair from Dragon Age: Origins. While courageous and noble and all, his main party gifts are action figures. The Feast Day DLC gives him Grey Warden Hand Puppets that you can watch him play with.
Alistair (upon pulling out his dollies): "What's that? You want me to be quiet?!"
- Possibly Lord Roth from Infinite Space. Well, he is really competent that he becomes one of the top zero-G dogs in the universe, but personality-wise, he fits this trope.
- Taizo Hori from Mr. Driller and Dig Dug, is as energetic and dumb as a little kid, he even throws childish tantrums when he doesn't get proper recognition. Quite funny considering he's the oldest of the main cast being 45 years old It's probably the reason why his wife left him.
- SoulCalibur's Xianghua has a prone move that for all intents and purposes, appears to be a childish tantrum.
- Any adult Sim on The Sims 3 that has the "Childish" trait will play with toys, get along better with children, hate doing adult things (like working), and like to make faces at people. This trope also counts when it is a Teen or Elder sim. They are also able to fish in swimming pools.
- Meredy in Tales of Eternia. Ten years before the start of the game, she witnessed her father Balir's murder at the hands of a group of people he and Meredy's mother Shizel thought they could trust (including Shizel's brother Hyades). In her grief and anger, Shizel was left open to possesion by Neried and annihilated everyone in the area save Meredy and Hyades due to them having the same potential to be possesed. In response to all of this, Meredy essentially froze her mind in the state of a six year old's.
- Hometown Story has Sue the witch who's two centuries old, looks like a young teenager at the oldest and is frequently seen playing with the two young boys among the characters, one of whom has a crush on her. The game has an event during which the village lumberjack considers giving wood to the kids (including Sue) so they can have a bonfire, but only feels comfortable doing it if they are supervised by the Player Character (an ordinary human older teen or young adult).
- In Final Fantasy VII, Barret is 35 and generally interacts equally with Cloud, Aeris and Tifa, who have an average age of 21 (and act like it). This is especially notable with Cid, who's 32 and makes it clear that he views Cloud, Aeris and Tifa as kids.
- In Phoenix Wright: Ace Attorney, Maya Fey very much fits the bill (even when she is 17 in the first game), most prominently her complete obsession with children's television programs, which Phoenix notes in the 3rd game. The fourth game hints that she hasn't changed in nine years, given that she would be 26 by that time. In the first game she argues with a seven-year-old about said children's programs, and in the second and third games she's often in the company of her eight/nine-year-old cousin, highlighting her childish personality.
- Taiga Fujimura in Fate/stay night., who despite being an adult is almost always in the company of people younger than her, and her maturity level is the same as theirs if not younger. This means that she gets kicked out of the route around halfway through each time. Notable childishness: berserker fury over being called Tiger, challenging Saber to a fight and bursting into tears when she loses, the most overboard expressions in the game, and switching the labels on soy sauce and oyster juice. Notably, the visual novel shows that she only acts immature at home and is generally much more competent as a teacher.
- Dangan Ronpa has Yasuhiro Hagakure, who's 20 years old while the other main characters are high school students (he was held back a few years). Despite being the oldest of the students, he's probably even more ditzy and immature than the rest of them.
- In the Lighter and Softer Watchmen parody, G-rated Watchmen, the Watchmen characters are given the behavior and personalities of young children. The authority figures are treated as the "grown-ups", despite being around the same age as the main characters.
- Pinkie Pie in Slice of Life, who ends up being a confidante to the Cake twins as a result.
- It's deconstructed in My Little Pony: The Mentally Advanced Series: being One of the Kids does not make Rainbow Dash a good babysitter, as she encourages and takes part in kids' shenanigans, no matter how bad an idea it is. She got banned from babysitting her cousin's daughter because they ate ice cream for dinner and it ended with Dash hospitalized and the kid sick from the ice cream. It's later revealed that she has a court order barring her from being alone with children because of it, which nearly got her in trouble in Rainbow Dash Presents: Bittersweet. She got off because she wasn't intentionally watching the foals alone due to them deciding to run away from home.
- Pinkie Pie is one in Friendship is Witchcraft however it's due to her traumatic foalhood. Her parents died when she was young and she was sent to an orphanage where she was treated poorly and emotionally absused for being an Earth Pony and due to her Romani heritage. Twilight questions her about this when she goes Trick-or-Treating with foals.
Pinkie Pie: I'm just a kid! I fit in!
Twilight Sparkle: Pinkie Pie, didn't you already have a childhood?
Pinkie Pie: No.
- Captain Sunshine from The Venture Bros. is a superhero whom is a parody of Batman. Everyone thinks he's a pedophile due to the way he acts around young boys; as it turns out he's just a big Manchild who's lonely after his first Wonderboy died.
- Moral Orel offers a rather creepy version of this. Doughy's parents, who must be somewhere in their thirties, act, and dress like they're still in High School. This extends to their parenting,(If it could be called that) where they treat Doughy more like an Annoying Younger Sibling, than a son.
- The King from Jelly Jamm possesses no more maturity than his frequent playmate, Rita.
- Invoked in an episode of Rocko's Modern Life where Mr. Bighead suffers a midlife crisis and attempts to get in on the fun with Rocko and his friends.
- Jake from Adventure Time: okay, he's a dog, but he's twenty-eight in dog years. His best buddy is a fifteen-year-old.
- Played for Laughs on Young Justice, when Captain Marvel is tasked with mentoring the Team. He adores them and repeatedly tries to hang out with them on missions and social events, while they treat him as either an intrusive spy from the Justice League or, at best, the way most teenagers would treat a grown man who acts like a ten-year-old. The joke, of course, is that Captain Marvel is a ten-year-old Henshin Hero who transforms into an adult superhero, but neither the Team nor most of the Justice League know that.
- My Little Pony: Friendship Is Magic:
- Vague Age aside, Pinkie Pie clearly has shades of this as demonstrated by her trick-or-treating in the Halloween Episode "Luna Eclipsed" and getting asked if she's too old for this.
Pinkie: Too old for free candy? Never!
- The second season episodes "Baby Cakes" and "A Friend In Deed" serve to demonstrate the more problematic aspects of this trope. In the former, Pinkie lacks an understanding of the responsibilities attendant to childcare, initially seeing the newly born Cake Twins as little more than new playmates and taking a full episode learning about what it means to actually be responsible for them. In the latter, she shows a strong lack of understanding about the importance of privacy and property, rooting through Cranky's possessions and forcing her way into his home, frequently ignoring pleas to stop.
- Vague Age aside, Pinkie Pie clearly has shades of this as demonstrated by her trick-or-treating in the Halloween Episode "Luna Eclipsed" and getting asked if she's too old for this.
- Johnny Bravo is a main-character example, who interacts with the neighborhood kids like he's one of them. He also lives with his mother like a child, but he's clearly an adult.
- Richard in The Amazing World of Gumball. He spends a lot of time with his sons and sometimes his daughter, mostly because he's a Manchild Bumbling Dad who doesn't have a job (and actually can't have a job or the universe will be destroyed).
- This appears to be Uncle Grandpa's job. If he's interacting with someone he doesn't live with, he's probably helping a kid.
- The main character of BoJack Horseman is a deconstruction of immature grown men. Being in his fifties but having the mind and outlook of a teenager means he has an uncanny ability to relate to younger people and bond with them, but it also makes him irresponsible and much, much more susceptible to doing really silly things that put him in a bad spot or ruin his relationships with others. Plus, by hanging out with tweens and teens, people see him as a creepy, pathetic old man (which they might be right about).
- Websites with a membership system and a community that caters to something intended for a younger audience will inevitably have some members who are much older than said intended audience yet enjoy regardless. Since they tend to act the exact same as everyone else and are treated as such, it often comes as quite the surprise when the guy with the avatar of SpongeBob SquarePants doing the Impossibru face who always makes typos turns out to be in his forties or something.