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.
See also Wise Beyond Their Years. The difference between this situation and Wise Beyond Their Years is that One of the Kids is rarely treated as any kind of authority; because of their casualness they often do not even earn special respect.
When examples of this trope show up in shows where they are above the Competence Zone of the rest of the cast, they tend to be teachers, which often yields Sensei-chan.
Related to, but not to be confused with, Man Child.
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Anime And Manga
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.
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 big guy Jumbo, who starts a one-sided competition with ten-year-old Miura over visiting Hawaii. Yotsuba's father sometimes acts like a teenage boy with Jumbo, but that may just be because they're childhood friends with a lot of history together.
Perhaps best exemplified in chapter 53 when Koiwai leaves Yotsuba alone with Jumbo to house sit while he's away. Yanda ends up stopping by to mooch some hot water and by the time Koiwai comes back the three of them are having a water balloon fight inside the house.
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.
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.
Sein of Magical Girl 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.
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 Kuuchuu Buranko 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.
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.
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.
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—or is it supposed to be middle school? If it's supposed to be middle school, it's Dawson Casting; if high school, then not.
Middle school in season 1, high school in subsequent seasons (even though it's all in the same school building).
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.
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.
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 creepy/sad/whatever part? Even if she is genuinely friendly and never holds any ill intention, you must remember that she's 28 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.
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.
Danganronpa 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 SofterWatchmen 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.
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 Man Child 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.
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.
Captain Marvel from Young Justice, who despite having the wisdom of Solomon, is nonetheless a ten-year-old boy who can use a grown up's body. He absolutely adores the team he was assigned to mentor, and often feels a bit disheartened that they don't treat him like a friend, but like any other adult super-hero.
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.
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.