A whole rafter of British sitcoms started spawning in the The '90s and well into the new millenium, where the central characters were Man-Children and their Distaff Counterpart equivalents who were alone, adrift and floundering in a hostile world. The central character of Bad Education, for instance, a completely inept teacher trying to get down with the kids and failing; Sharon Horgan in Pulling and her circle of friends were the female equivalent. Other examples might include Him & Her, Simon Pegg and his friends in Spaced, and many others. Home-grown comedy on the younth-orientated BBC3 channel depended on this trope.
Max from 2 Broke Girls has shades of this while she's not as immature as a child, she has a childlike enthusiasm for things like drugs and candy. She's essentially what Amy Poehler's Saturday Night Live character Caitlin, a hyperactive 10 year old, may be like 15 years later. She also could possibly be a compulsive liar as many of her stories seem way too farfetched to be true and something a child would tell.
While all the characters from Arrested Development fall into this trope to some degree (the clue's in the title, after all), there are a couple of stand-out examples:
Buster is the most obvious: in his early thirties when the series begins, he nevertheless acts like a literal child most of the time, and is treated as one by most of his relatives. He not only still lives with his mother but sleeps in the same bed as her as often as not, and the amount of control she exerts over every aspect of his life would be weird for any kid over kindergarten age, let alone an adult approaching middle age.
GOB, despite being the only member of the Bluth family living semi-independently and maintaining a (slightly) more mature facade than his brother, nevertheless can only identify two emotions by name ("envy" and "hungry"), and entirely fails to ever take the potential consequences for his actions into account. Poignantly, as the series goes on it becomes increasingly obvious that his status as The Unfavorite compared to his three siblings is responsible to a large degree: his parents openly admit to loving him the least, and his desperation for the approval of anyone in his family leads him to unquestioningly do whatever he's told.
The Beverly Hillbillies: Jethro Bodine, in spades. Although his emotional and mental immaturity is owed to his lack of education (he's proud to be a sixth-grade graduate) and awe at being introduced to the Beverly Hills lifestyle.
The Big Bang Theory: While all the characters indulge themselves in childish things, Sheldon insists that someone take care of him the way his mother would when he's sick (or even just homesick), locks himself in his bedroom where no one else is allowed when he's furious, curls up into a crying ball of sad on his bed when he's embarrassed, runs away from home when he's upset, is practically traumatized by the sound of people arguingnote Although this one may be perfectly justified, as his parents often got into arguments when he was a child and it is also implied that a lot of the arguments were exceedingly violent (he mentioned that his father would skeet shoot his wife's collectible dinner plates from the roof, and that his mom planned to put glass shards in his father's meatloaf), even when they said to Sheldon that they stopped fighting, and is stubborn and petty beyond all reason. The only thing on this planet that can force him to behave rationally when he's angry or depressed is an order from his mother, who Leonard calls "Sheldon's Kryptonite". He also responds positively to being patronized by Penny, especially if he gets a toy robot and a comic book out of it. Lampshaded by Bernadette at one point, after she successfully got him to go to bed by first reasoning with him about the effects of lack of sleep, and then simply treating him like an overgrown child and sending him to bed.
Bonanza: Several episodes have featured characters who were mentally "slow" some villains, some sympathetic characters, others one-off good guys the Cartwrights aim to help. One of the best-known examples was "The Ape," from the show's second season, where a large man named Arnie has traces of autism, is unable to read social cues and (more worryingly) has a fierce temper. His fierce determination to win over a barmaid that has no interest in him is what eventually leads to his downfall, despite Hoss' own determination to help focus Arnie and mentor him into a farmer.
Detective Jake Peralta is a very good detective, but he's almost incapable of acting like an adult.
Sgt. Terry Jeffords: The one mystery [Jake] can't solve... is how to act like a grown-up.
In many ways, Detective Amy Santiago is a female version of this trope; she's just a different kind of child. Where Peralta acts like the high-school class clown who never learned maturity, Santiago gives off the impression that she's still running for high school valedictorian.
Jake: God, you must have been the worst fourth-grader ever. Amy:(smug) Joke's on you, I skipped fourth grade.
Richard Castle is this trope in spades. He sets up laser tag in his house, plays with remote-controlled helicopters and other toys, buys property on the moon, and often has to be put in line by his teenage daughter. However he is capable of acting maturely, he generally just chooses not to.
Buffy the Vampire Slayer has Xander Harris. He sometimes behaves like a child and comes very immature. In a sequel in the third season, an enchanted chocolate is distributed in Sunnydale, which mentally brings the adults back to the level of children and teenagers. Xander says he does not feel any change even after three plates of chocolate.
The villains from the sixth season, Warren, Johnathan, and Andrew are simply nerds who have decided to be evil for fun. The three often behave like children in their performances, starting in an episode of the actor who was the best James Bond. In another episode, Andrew paints a picture of the very conspicuous Death Star on the car of the three, with whom they want to observe Buffy. All three always make references to pop culture, and argue like children.
The Christmas That Almost Wasn't: Prune exhibits this, even before rediscovering his forgotten childhood. He's extremely petty in his actions and outlook, even breaking toys and blaming it on Santa so the rent money all goes to repairs.
Morgan Grimes of Chuck has strong shades of this in the first two seasons, although around the time he gets promoted to assistant manager of the store he finally begins growing up. He never completely abandons his love of video games, comic books, toys, and other such interests, but his maturity level from season 3 onward is strikingly higher compared to the beginning of the series.
Iron Fist is stunted emotionally due to his childhood trauma and being taught to suppress his emotions by the monks who raised him. Because of this, he often acts childish in front of his peers (such as switching seats during a meeting so he can sit by Joy), not understanding social norms (such as going into Colleen's dojo to spar), or losing his temper (such as when he fresked out at Ward for his bullying as children).
The First Doctor. He sees himself as a dignified old man but his maturity level is closer to that of a fourteen-year-old: loving showing off to girls, being surly to authority figures, struggling with social skills, incredible self-centredness, gleeful meddling and boundless curiosity (not to mention his somewhat vertical relationship with Ian and Barbara and his incredibly parallel relationship with Vicki). Later canon pretty much states the "old man" aspects of his character were actually just a childlike attempt to look important. All this said, all of his later incarnations trust him to the point of seeing him as an authority over them.
The Fourth Doctor. One of the most dignified, mature and authoritarian Doctors of all the Third was immediately followed by probably the most blatantly childish and anti-authoritarian of the lot. He offers everyone sweets, plays with children's toys, doodles cartoons of people he dislikes, loves playing and running around and getting attention, tends to sulk when he can't get his way, and has childish Character Tics that make him bizarrely cute despite being huge and imposing. He's sensitive and vain about his age, constantly lying about it to seem younger. He absolutely detests authority and is willing to go anywhere, do anything to avoid taking orders again.
The Eleventh Doctor plays the age card less than previous Doctors did during an argument, and seems to even forget his decrepitude at times. The War Doctor isn't amused: a senile git of a man, Eleven has retreated from his grim past into a world of childlike frivolity. This changes once he saves Gallifrey in "The Day of the Doctor": He pointedly chooses to stay behind and age into an old man on Trenzalore, as if unconsciously deciding to "grow up".
The Twelfth Doctor is rude to almost everyone around him, but sometimes shows a more caring side, and when he is alone he spends most of his time brooding. In essence, he is a rebellious teenager in the body of a grown man.
The Thirteenth Doctor is as hyperactive and scatterbrained as the Eleventh, and has some very childish tendencies as well, such as stomping her feet when she's angry, jumping up and down when she's excited and pouting when she doesn't get her way.
The Dukes of Hazzard: James Best once said of Sheriff Rosco P. Coltrane the character fitting this trope that Rosco is like a 7-year-old child who loves initiating car chases. Indeed, much of Rosco's behavior is that of a 7-year-old. While that of Hazzard County Commissioner J.D. "Boss" Hogg (Rosco's boss, confidant and partner in crime ... and his brother-in-law) is similar to the 10-year-old big brother.
All six members of the main cast of Friends tend to act childish:
Ross is the worst one of the group. He usually resorts to petty revenge to get back at Rachel whenever they break up from their on again off again relationship (dating women just to make Rachel jealous) and gets into childish fights with his sister, Monica, such as hogging the TV or calling her childish names. For the latter, Ross thought his squabbles with Monica as they grew up together was harmless fun and that is why he kept doing it well into adulthood until Monica confessed that she hated Ross when they were kids because of his bullying.
Joey is almost as bad in this regard. He constantly mooches off Chandler (with everything from rent payments to acting classes), he refuses to maintain a steady romantic relationship even when he's well into his mid/late-30's, and he's hopelessly idealistic about his acting career despite clearly being a pretty lousy actor. As the show went on these traits were Flanderized for him, and by the final season he was buying Cabbage Patch Kids dolls, being entertained by a book intended for preschoolers, and had a written letter of his being mistaken as one written by an actual six-year-old.
Rachel has a childish need to be the center of attention, refuses to take responsibility for her own actions, and constantly whines to her friends about her personal problems. Not to mention everything she does to sabotage Ross's relationships and all the arguments she gets into with him.
Chandler constantly makes immature jokes and (along with Joey) often gets caught up in activities that are meant for children far below his age group.
Phoebe has a childishly naive outlook towards life (even with her crappy childhood) and still believes in fantasy figures such as Santa Claus.
Monica is the most mature, except on the few occasions where she becomes a Control Freak and the times where she gets into fights with Ross. Because of how competitive she is she also often throws huge tantrums when things aren't going her away during any sort of game she plays against one of the others.
On Fringe, Walter Bishop is a curious example of this. As a young man, he was a normal, mature adult. But then his partner cut pieces out of his brain and he spent 17 years in a mental institution straight out of Victorian England (okay, not quite that bad). The new Walter Bishop, while he retains his prodigious intelligence, is a Cloudcuckoolander who cannot manage the basic necessities of life on his own, and is dependent on his adult son and his FBI-agent-turned-lab-assistant to provide for his needs. While his foibles are often played for comedy, later episodes also play up the pathos of his situation; unlike most Man Children, Walter remembers having been capable of living on his own, and his humiliation and sense of loss are keenly felt.
Joey Gladstone in Full House loves cartoons, junk food, and having fun most of the time, and is in his 30s. By the time of Fuller House, he's in his 50s and hasn't changed a bit.
Rory: You'd have to cut him open and count the rings.
Rose Nylund of The Golden Girls is a rare female version at times. It seems that her hometown of St. Olaf has produced a fair number of these, although it can be hard to tell from her stories who is a Manchild and who is just a complete idiot.
On Good Eats, Alton helps out a 32-year-old man who has his mother come by every morning and make him breakfast, at the mother's request. He eventually learns to at least cook one meal for himself, but his mom still does his laundry for him.
Good Luck Charlie: Despite being the oldest sibling, PJ is very childish and immature for his age since he still watches kid shows, still believes in fairy tales, and is afraid of things that only children fear (such as clowns, the dentist, etc.)
On Grace and Frankie, Frankie is an extremely rare female example, mostly applicable because she is in her 70's and the usual female tropes focus on much younger women. Frankie throws tantrums when people don't do what she wants, she sings loudly when people say things she doesn't want to hear and will pull stunts like locking her family out of the house (or locking herself in) when she is mad at them. She is also financially irresponsible and has relied heavily on her (now ex-) husband and sons to handle the practical matters of life.
Season 6 of How I Met Your Mother introduces Becky, Robin's co-anchor on Come On, Get Up, New York!, who talks like a baby and has the personality of an 8 year-old girl. Ted was basically her "dad" when he briefly dated her.
iCarly: Spencer is a sculptor for a living, often making whimsical and sometimes weird sculptures that usually end up catching on fire or exploding before being perfected. He loves light-up socks, has a pet goldfish (that he often forgets to feed, resulting in another pet goldfish), and once went to law school for three days.
The most physically intimidating of the Impractical Jokers - heavy-set, scowly Q - had to don a nightshirt and tug around a teddy bear in an Ikea. Surprisingly, he managed to get a customer to tell him a bedtime story.
The IT Crowd: Moss is a man in his thirties who acts and speaks like a 4 year-old most of the time, doesn't swear, still lives with his mother and sobs uncontrollably when confronted by bullies...
Roy to a lesser extent.
Ellen May in Justified. The relationship between Delroy and Ellen May resembled that of an abusive father and child. Later, her relationship with Ava took on a similar dynamic.
Kamen Rider Ex-Aid: Nico Saiba is 18, but you couldn't tell from the way she acts and dresses. She is a Troll, who decorates her room with Tastes Like Diabetes and grates on people's nerves with her cheerfulattitude. Later parts of the story put a twist onto this as more of her act becomes pretending to be a WomenChild out of habit, love for trolling people and need to continue her reputation because she doesn't want people to know she grew up.
Downplayed example in Kamen Rider Build: Sento Kiriyu is a serious adult rightfully called "hero of justice" as Kamen Rider Build. He is also a science geek with a massive ego, who tried to test his invention on people around him and he agrees with his friends that the more fitting version would be "narcissistic, conceited hero of justice".
Doug Heffernan from The King of Queens. Whenever he's having an argument with Carrie, he often goes to extreme lengths for petty revenge, often injuring himself or worsening the conflict. He has been shown to have a very juvenile sense of humor on several occasions, and one time fought with his father-in-law over a toy in his cereal box, just to name a few things.
Lenny and Squiggy from Laverne & Shirley are this trope. While they do have some adult ambitions like getting girlfriends and starting a talent agency, they still have the same mindset as kids and often do childish things like drinking chocolate syrup and watching cartoons.
The Law & Order: Criminal Intent episode "Cruise To Nowhere" focuses on a young (about 20 years old) poker genius named Joey Frost, who acts like a petulant child and whom Goren describes as having the emotional maturity of a ten year-old. It eventually turns out that there's a sympathetic reason for this behaviour: a horrible childhood.
Goren himself fit this trope in the early seasons. Witness, for example, his glee at discovering a synthesizer that says things like "Oh, baby!" when you hit the keys.
Sometimes in later seasons, too, such as in 'Vanishing Act' when he reacts with similar glee at the various magic trick thingies.
On Leverage Parker has shades of this given that she was somewhat emotionally stunted by becoming a thief at an extremely young age.
Jacob from Lost began as an immature, emotional man with little social skills, plus he lived with his adoptive mother for nearly thirty years. Centuries later, however, Jacob's outgrown his previous phase.
Has he though? "You've got ink on your forehead."
Stuart of Mad TV. Although the character is supposed to be an actual child, he appears as one of these because he is played by an adult man.
Radar in some seasons ofM*A*S*H. He does have some growing up to do in "Idols", and, as a final show of growing up, leaves his teddy bear behind when he goes home.
Master of None: Both Dev and his best friend Arnold. In the pilot, they attend a baby's birthday party because it has a bounce house, and are inordinately entertained by baby toys.
Vince in The Mighty Boosh is constantly eating sweets, writes in crayon and has an imaginary friend made out of bubble-gum called Charlie.
Adrian Monk from the eponymous series Monk can be this at times. He tends to cringe and whine when faced with one of his multiple phobias. He's still obsessed with his childhood favorite show The Cooper Clan at least until he finds out the truth about the stars in Season 8. One particular episode has him totally revert to acting like a nine-year-old as a result of hypnotism. He snaps out of it by the end because Status Quo Is God.
Mr. Bean sleeps with a teddy bear. In the first episode, he puts two dolls on the table when he sits down to take an examination. In general he is of the more common, Fish out of Water type, although his strangeness goes beyond childlike and into the realm of truly bizarre.
The Munsters: Herman Munster. He enjoys watching children's programs, and frequently throws foundation-shaking temper tantrums.
Roger Bailey Jr. of the series My Family is an irrationally optimistic character who has a somewhat infantile perception of the world, as a result of his doting mother's over-parenting. His idea of a fun stag night is making balloon animals and he is an ardent collector of Lord of the Rings figurines. It is later revealed that his mother still cuts his food up for him.
In the same series, Earl had caused his teenage babysitter to become pregnant by poking holes in the condoms she and her boyfriend were using, because he had a crush on her and he was jealous. The couple is still together, having gotten married as a result of the girl's pregnancy, and their son is now grown up. He, however, is a mess; he dropped out of school, lives in the basement, is whiny and demanding, and uses a Jar Potty so he doesn't have to come upstairs to use the bathroom. Earl decides that the way to cross "Got the babysitter pregnant" off his list is to help him grow up. It's actually Randy that ends up doing it, though.
April of the same show fits as a Woman Child. Since she and Andy are married and live in the same house, which they never cleaned, repaired, or paid for, Ben has to babysit for them.
Tom and Jean-Ralphio also fit. They started Entertainment 720 together so they could look cool. The business tanks into debt in about 2 months because they didn't know that companies have to make money.
Leslie fits, too. She is cheerful, hyperactive, and a little naïve. Not to mention her fondness for candy and dislike of vegetables. She is competent, however—indeed, very competent.
Connor Temple from Primeval also often behaves childish and immature. He is constantly joking, and often makes references to pop culture. It is even a running gag, that he always gets into trouble, and must be saved. In the later seasons, however, he becomes a little more mature.
Shawn Spencer from Psych, especially in the episode Ghosts: Shawn responds to his mother's unexpected appearance by hiding out in his old bedroom (though this has more weight to it: it's bringing up memories of the divorce his folks went through, which led to a Broken Pedestal with his dad.) While on the phone with Gus, a conversation during which he becomes enamored with an unopened box of Shrinky Dinks, a teen magazine in pristine condition (which he tries to read to Gus over the phone) and playing with an old Furby. The latter one is especially telling as he would have to be 21 at the youngest to have previously owned a Furby.
In one episode, after freaking out about his Nintendo DS being stolen, his girlfriend tells him to stop acting like a child and he responds "I'm not acting."
Justified, in that there was an engagement ring (that's a family heirloom no less) for said girlfriend hidden in the Nintendo DS.
Another is where he answers said girlfriend when she asked how his dad went, he mentions watching Phineas and Ferb, having a snack and then napping.
It's hinted that a good deal of Shawn's childish moments was due to the strict upbringing by Henry; while he did have a fulfilling childhood that his dad was an active part of, he still missed out on opportunities to do things he wanted (his father heavily disapproved of superhero comics for how they make cops look for example). Furthermore, most of said acitivites were molded to serve as learning experiences for Shawn to become a cop. So it could just be that Shawn's childish acting out is a way to indulge in being free to do what he wants.
To a lesser extent, but still prevalent, is his best friend Burton Guster. While he does play the Straight Man to his antics, holds a respectable job and an overall more serious man, he has plenty of moments where he indulges in simple fun with Shawn. Heck, he has even been seen in pajamas with trains on them. He loves bunnies much like Shawn does and is actually pretty sensitive emotionally. He and Shawn are pretty co-dependent on one another (though in Shawn's case, it's because Gus was one of the few constants in his life.)
Raven's Home: Despite having gotten older around their 30s, Raven and Chelsea have not matured at all since their teenage years. They are very childish and immature single mothers who can't handle the responsibility and seriousness of being parents. In fact, their own children are practically the ones raising them.
'School of Rock has Mr Finn. He lives with his father and is a failed musician who was kicked out of his former band. This does work to his advantage though, since when he starts teaching, he's better able to relate to the students by being at their level.
Many of the Sesame Street Muppets are this. They do some classically "adult" things (Bert and Ernie share an apartment, Big Bird lives semi-independently in his nest, Grover has had many jobs). Yet developmentally, they represent the ages of the target audience (Big Bird is developmentally 6, Grover is developmentally 4, and so on, according to Word of God).
Sherlock in Sherlock. He does very, very, very strange things to solve cases, and...well, there's a lot that would qualify him for this category.
Caitlyn on Saturday Night Live for the same reasons as the above is supposed to be a 10 year old girl, but was played by then the well into her 30's Amy Poehler.
Schemer from Shining Time Station is a full grown adult and doesn't know how to take care of himself. He also doesn't seem to understand the value of a dollar, as he thinks a nickel is a valuable amount of currency. (Even in the '90s, any self-respecting arcade machine would cost at least a quarter to use.) Taken Up to Eleven in "Schemer's Alone", where his mother goes away overnight.
Schemer: "Take out the trash, make your bed, say please and thank you." I mean what does she think I am, some kind of mature adult?
In one episode of The Suite Life on Deck called "The Starship Tipton", George Takei played a descendant of London named 'Rome Tipton'. He is usually seen with a teddy bear and acted very much like London. He was even still in school.
When Castiel is dealing with Heavenly matters, he's a stoicBadass Longcoat. When he gets isolated among human beings, however, he descends into this at times, like when he goes to a brothel, suddenly craves meat, or...
Castiel: "I found a liquor store... and I drank it." (falls onto Sam)
Dean Winchester is this whenever the writers decide to take a break from angsting him. For example, he gets super-excited about classic monsters like a proper werewolf, and makes movie references non-stop.
In Bad Day At Black Rock, while his run of luck allows him to have incredible reflexes and aim, Dean says "I'm Batman" after taking out an assailant, and looks genuinely disappointed/hurt when Sam responds with a "Yeah right, you're Batman".
In Season 6, Dean also shows his childish excitement of classic Western movies and the opportunity to go to the good ol' West.
Neal Caffrey from White Collar shows shades of this. When Peter tells him it's wrong to have nice things you haven't earned his reaction is a childish "why not?". Also, whatever mistakes he makes always seem to because he can't keep his feelings in check.
Multiple characters also lampshade his tendency to fail to completely think through the consequences of his actions, and inability to grow up. His ex/current girlfriend Sara tells him that he has a tendency to live with his head in the clouds, and Agent Peter Burke actually calls him Peter Pan several times early in the series, and tells him he can either be a con or a man, not both, in an effort to encourage him to get his life on track. In fact, he and his wife often give him advice that a parent normally would, despite the fact that they are not actually old enough to be his parents. Because he's so immature, in some ways, and didn't have much of a relationship with his real parents, this still works.
Mozzie (exasperated, after Neal steals a painting despite the fact that it would be obvious that it was him): You're like a child.