Played with in The 40-Year-Old Virgin; the main character is, as the title suggests, not sexually active, and also dresses in a rather buttoned-down fashion, has a typically childish hobby of collecting comic-book action-figures and is slightly naive and inexperienced, coming across on the surface as being one of these. However, on the whole he's actually managed to get his shit together a lot more successfully than many of the supposedly more 'experienced' men and women around him, and generally comes off as being a lot more mature, well-rounded and wise about life than them.
The adult villain from The Adventures of Elmo in Grouchland's big deal is that he doesn't like to ever share anything, including a blanket he stole, for god's sake!
The title character in both versions of Arthur (1981), thanks to wealthy upbringing and a multi-million dollar inheritance, has never needed to hold down a job and is now The AlcoholicFun PersonifiedTabloid Melodrama shame of his by-and-large humorless family as an adult. While he uses his money to enjoy very adult activities, his townhouse also has various childhood pleasures (a giant model train set, a pinball machine, etc.) in the 1981 original, and in the 2011 remake his Establishing Character Moment is a joyride with his chauffeur in the Batman Forever Batmobile. His valet has become his Parental Substitute on top of all this. Ultimately, falling in love with a working-class woman and the threat of losing his money if he chooses her over an Arranged Marriage forces him to gain some measure of maturity to find true happiness (more so in the remake, where the original saved further Character Development for the sequel). The trope is more pronounced in the 1981 original because Dudley Moore was 46 at the time, whereas the remake's Russell Brand was 34. The Expository Theme Tune that appears in both versions discusses this: "Arthur he does as he pleases/All of his life he's mastered choice/And deep in his heart/He's just, he's just a boy".
Back to the Future: In the original timeline, adult Biff Tannen acts exactly like his teenage bully self from 1955, having no reason to ever mature; he forces George to do his work and takes advantage of him. In Part II, this crosses over to Psychopathic Manchild in 1985-A when he's still just as bratty and immature, but now has the wealth and power to get everything he wants. The original Biff never actually killed anyone (though he did try to run Marty down with his car, twice); 1985-A Biff has no misgivings and demonstrates it.
Thurman Merman from Bad Santa 2 he's now in his early twenties and acts exactly the same as he did in the original film, he's still very innocent, believes in Santa Claus, sleeps with pink stuffed animals, and is dependent on others.
The title character of Baby Doll, an Innocent Fanservice Girl who is also as obnoxiously emotional as a child. She even still sucks her thumb. However, being the underage bride of a man twice her age has made her more cynical and disillusioned than most examples. Plus, she consciously represses her own sexuality.
Daisy Kensington from Barefoot was raised in isolation (by a schizophrenic mother), and everything she knows about the outside world comes from watching TV. When her mother dies, Daisy winds up in a mental hospital, but the film's protagonist breaks her out and winds up having to teach her how to drive, how to flush an airline toilet, that it's not good manners to go to a strip club and dance on stage with the strippers, etc.
Selena Kyle in Batman Returns starts out as a downplayed version of this. Unlike most examples she lives independently, holds down a job and generally acts like an adult, albeit very timid and insecure. However, her home is bright pink and cutesy and she's shown to own dozens of stuffed animals and a dollhouse.
Chance the Gardener in Being There; in the movie version his maid Louise actually says "You're always gonna be a little boy, ain't cha?" when she leaves after the death of the master of the house. It's stated in the book and heavily implied in the film that he is mentally challenged; the twist is that most of the other characters don't recognize this, making the character a Trope Namer for similar mistaken identity situations. Peter Sellers played him in the film; he initiated its making because he identified with the character so strongly when he read the book. For better and worse, he was a Real Life example of this.
Josh Baskin comes off this way in Big, he really is a 12-year-old boy in the body of an adult due to a wish he made to become taller so he could ride a roller coaster at the carnival. It gets Squicky when, in an attempt to prove to his mother he's really her son despite having a 30-year-old's physique, he briefly pulls down his pants to show her his little-boy underwear and the birthmark on the back of his left knee, and she naturally freaks out.
Adam Webber from Blast from the Past was raised in a bunker with no human contact outside his parents. As a result, he comes across as childish in social interactions.
When the two of them aren't killing gangsters, Connor and Murphy McManus, The Boondock Saints, are prone to bickering and tussling like 10-year-old boys.
The Bronze: Hope is one at the beginning. She mooches off her dad, while living in his house, stealing from his mail bag (a federal crime, as he notes) and acts generally like a spoiled kid. Her dad enables and tolerates this at first, but finally tries to push her into maturing (which eventually works).
Billy from Buffalo '66 is gradually revealed to be this. He doesn't like girls, he makes up bizarre stories, he bullies his friend, and is naive enough to get in trouble with gamblers.
Charlie and the Chocolate Factory has Willy Wonka, who comes off as eerily similar to Pee-Wee Herman in this adaptation. But while Wonka appears to be completely unselfconscious of what a manchild he is (at one point in the film, he argues that he was never as small as a child, because he remembers being able to reach his head to put a hat on top of it), unlike in Herman's world other people notice and are more than a little squicked by him.
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.
Fatty Arbuckle in many of his films. In Coney Island, he is introduced at the beach, shoveling sand into a plastic bucket with a toy shovel.
Wade Wilson in Deadpool (2016). He makes goofy crayon drawings of his revenge schemes, proposes to his girlfriend with a ring pop, and generally acts silly. Takes a turn for Psychopathic Manchild after his transformation-via-mad-science-torture and subsequent Roaring Rampage of Revenge.
Dinner In America: Patty is a rare female version. She's 20 years old but dresses and behaves like a young teenager. She lives under the thumb of her parents, asking their permission to attend a concert. It's not exactly clear how she ended up this way, though she takes a fistfull of mind-affecting medication every day for unspecified reasons.
Buddy from Elf is over 35-years-old and still acts like a 6-year-old and has a very infantile view of the world; this is due to being raised by elves from infanthood; Buddy even notes that he doesn't know many other people who "share my affinity for elf culture."
In the American version of Fever Pitch, Jimmy Fallon's character is called a manchild by his girlfriend, but all-consuming passion for the Red Sox aside, he's a comparatively mild example: he is good with women, has a job as a well-respected junior high math teacher, and has his own apartment.
In Fear, Inc., Joe Foster is an unemployed slacker who sponges off his wealthy girlfriend: lounging around her luxury home, playing video games, pranking the neighbours, and ignoring the list of chores she leaves for him to do.
In Finding Neverland, James Barrie is (mostly) capable of taking care of himself, but he has an air of immaturity and childlike wonder about him, and he clearly has much more fun playing make believe with the Llewelyn Davies boys than he does socializing with adults.
Jason Voorhees from the Friday the 13th films is often characterized as this, especially by fans. Portrayed in the films as a hulking brute of a man who murders anybody he gets his hands on, his backstory is that he seemingly drowned at summer camp when he was a kid, though he actually survived and spent decades living as a hermit in an isolated shack deep in the woods. Having been born with a number of deformities that left him mentally impaired, and with nobody to raise him after his disappearance, he has the mind of a child, lashing out violently at anything that reminds him of the careless camp counselors who let him almost die.
Ghostbusters: While Peter Venkman is dry and cynical and Egon Spenger is clinically rational, Ray Stanz tends to have a lot of childlike enthusiasm in what they do. In the first film, Egon advised against buying the firehouse that ultimately becomes the Ghostbusters headquarters, but Ray's excitement while trying the fireman's pole forced their hand.
Ed from Good Burger. He is very Literal-Minded and generally acts like a dense yet lovable buffoon throughout the movie.
Jeff from Grandma's Boy (2006): he's over 30-years-old and still lives with his parents, he wears footy pajamas, he sleeps in a racecar bed with many stuffed animals while sucking his thumb, and spends most of his time playing video games.
Dodo from Heart of Dragon is an autistic 33-year-old man with the mindset of a 5-year old, with his only friends being a bunch of local schoolkids who often take advantage of Dodo. Such as having Dodo posing as an adult for parent-teacher meetings and to sneak into expensive restaurants.
Hearts Beat Loud: Frank, to an extent. Sam is shown to be the more responsible one several times, despite being younger. Frank is stuck in the past and trying to live out his band dreams, and is resistant to the upcoming changes about to come in his life. It's justified as this stems from his wife suddenly dying in a bicycle accident. Frank tells Leslie he copes partly by strongly bonding with his daughter Sam since this happened when she was little, and they played like kids together. Then eventually she outgrew him, and Frank was left behind to a degree.
Park Gang-du from The Host (2006) is the "lovable innocent" variation. He's upbeat, cheerful and good at heart, but he's a clueless and irresponsible single parent who lives with and works for his father. His siblings are constantly annoyed with him and the only one who understands him is his father. He finally matures, however, and reveals a fierce Papa Wolf side to him.
Marv from Home Alone is very childish at heart, contrasting Harry who's more overtly serious. His number one priority from the stolen houses are the toys and people's snow globes for his collection, and he floods their victim's houses because they're "The Wet Bandits" and "all the great ones leave calling cards". Needless to say he still manages to be every bit as violent and dangerous as Harry as he's fully willing to torture and kill Kevin, to the point he'd be a full-on Psychopathic Man Child if he wasn't so Affably Evil when not otherwise pressed.
Sam from i am sam, although he actually is mentally retarded. It's stated early in the film that he has the intellectual capacity of a 7-year-old.
The Incredible Shrinking Man: Downplayed. Even before he begins to shrink, Scott Carey relies excessively on other people. First, his well-to-do brother provides him with a job that affords him a comfortable lifestyle and also lets him borrow a boat for his vacation at the start of the film. Meanwhile, his wife handles all the household and domestic chores. It isn't until Scott is reduced to the size of an insect and trapped in the cellar of his house that he finally begins relying completely on himself. Scott needed to shrink in order to "grow up"!
The Island (2005): The clones, both male and female, were deliberately designed by Merrick and his staff to have the emotional and analytical aptitude of a child, as this made them easier to control. One can really see this in Lincoln's childish reactions early in the film to being denied bacon for breakfast or not liking the color of his clothes. It's only when he acquires more memories from his original that he starts to act more like an adult.
Inverted with the titular character in Jack due to accelerated aging. Jack seems like this, but he's not a man who acts like a child, he actually is a child and looks like a man.
James Bond: More so than any other actor in the franchise, Pierce Brosnan placed greater emphasis on his character being an emotionally stunted man. Bond fears commitment in a romantic relationship because he abandons his girlfriend Paris when he realizes that he's falling in love with her. Witness his childish glee as he "drives" his remote-controlled car during the multi-level parking lot chase scene. After Wai Lin compliments him on his motorcycle skills, his reply invokes this trope: "Well, that comes from not growing up at all." M describes his brand of charisma as "boyish," and Q says "Grow up, 007!" twice in exasperation. Alec asks Bond, "Why can't you just be a good boy and die?", Natalya accuses him of being "boys with toys," and Jinx scoffs, "You're a big boy; I figured you could handle yourself." M even vaguely serves as a maternal figure towards Bond, which further emphasizes his immaturity.
Laurel and Hardy: Stan, naturally. The appeal of the two is that they are both basically Man Children who can't get anything done without either causing an Escalating War or failing epically.
The Lifeguard has a female example where main character 29-year-old Leigh moves back in with her parents and starts dressing like, acting like, and hanging out with and having sex with a teenager.
Clara from The Light in the Piazza was kicked by a horse as a child and this left her mentally disabled. She's in her twenties but, according to her mother, has the mentality of a 10-year-old.
Marie Antoinette (2006) portrays Louis XVI as this way in his youth. He's awkward around Marie and obsessed with his hobby of locks. He doesn't try to consummate until months or years after their marriage, when it seems that the mechanics of sex are explained to him.
Thor and Loki are both mild examples in Thor. They're centuries-old alien demigods played by actors in their late twenties, but Thor starts out behaving like a frat boy (he improves with Character Development), and Loki, for all his cleverness, has the apparent emotional maturity of a melodramatic teenager (not to mention the daddy issues).
Peter Quill a.k.a Star-Lord from Guardians of the Galaxy has a childish fixation on pop music and his mother, and reacts to people taking his Walkman, the last connection he has to his mother, like a child's possessiveness over a toy. It helps that he was 8 when the Ravengers kidnapped him and raised him as their own, so he never actually had learned to grow up. His entire schtick is like a grade-schooler's fantasy of being a space hero.
Johnny Boy in Mean Streets seems to be emotionally and mentally stuck at age 14.
The main characters of Mystery Team are a group of "Kid Detectives" who still continue their exploits even though they're in their late teens and still having the mental age of seven. They are hired by a girl to solve her father's murder.
Ocean Heaven (a.k.a "Kung-Fu Superstar Jet Li's first drama") stars Li as a single father to a mentally ill, autistic adult son whose intelligence is on par with a third-grader.
Pain & Gain: Paul acts like a big kid at times. This is actually what makes him the most sympathetic of all three protagonists.
Paul Reuben's character Pee-wee Herman in Pee-wee's Big Adventure and Big Top Pee-wee. He seems to be an adult man who never grew up. He lives in his own house filled with toys. He has a girlfriend in each film, but shows a prepubescent lack of interest in them. He even has another Man Child rival. Other characters frequently refer to him as a "boy", though this could be just arrogance or condescension on their part.
Pixels: Both Sam and Ludlow, although Ludlow is a more severe example — in his forties, he has no job, lives with his grandma, has a Stalker Shrine of a computer game character, and spends his time making conspiracy theories over the internet.
Ben Barber from Ride Along is engaged to Angela and wants to prove he can be a dedicated police officer, but he's very impulsive and ambitious, has a penchant for comics and videogames so much that he calls himself "Black Hammer", and always complains like a Bratty Half-Pint when things don't go his way. James even calls Ben a "manchild" in the second film.
Dewey played by Jack Black in School of Rock is an immature rocker who has no life outside of rock. It helps him to find common ground with the kids.
Normally he's The Stoic and a genuinely intimidating presence but upset The Force Awakens's Kylo Ren (who's like 30) and he'll slash up everything in his vicinity in a violent rage while screaming like a banshee.
Step Brothers: Brennan and Dale are in their 40s and are very immature. They act like they're in their early teens at the oldest. They live with their respective parents, have no long-term goals in life, and are unemployed. When Brennan's mother and Dale's father marry, they fight constantly like children. When they finally get along, they excitedly run around the house and build themselves a bunk bed using hockey sticks for the wood. In one scene, they allow themselves to be bullied by actual children. They eventually start acting more responsibly after their parents divorce and force them to move out; they even get advice from therapists on how to act like adults.
Harry Langdon's comic persona. In The Strong Man, his character is utterly terrified when a woman makes what he thinks is a sexual advance (she's really trying to pick his pocket). He's similarly horrified by the sight of a nude model in an art studio. In general, he behaves with childlike innocence.
John Bennett from Ted is 35 and still acts like a child due to his lifelong friendship with the titular teddy bear that he wished to life as a child. He and Ted often engage in childish fights and are still afraid of thunder. However, once John realizes he has to be serious with his girlfriend, he finally begins growing up. He still never gives up on his friendship with Ted, though.
TRON has Kevin Flynn, who seems to have regressed to this state after getting kicked out of his company. When we first see him, he's the owner of an arcade and wowing his teenage customers with his virtuoso game skills. His office overlooks the arcade, Lora (his ex) shouts in frustration "Now, you see why all his friends are 14-years-old!"
Trippin, a 1999 coming-of-age comedy, has the protagonist's grandfather who at breakfast asks for someone to pass the bacon. The parents object to this pointing out that he has hypertension and is basically living off prescribed medication with it warned he could have a heart attack and die any minute if he's not careful. Nonetheless when refused his bacon Gramps chants "I WANT BACON!! I WANT BACON!! I WANT BACON!!" and proceeds to start spitting on the rest of the family's breakfast before the father finally gives in and gives him bacon just to shut up.
Gary King from The World's End is an idiot who still acts and dresses as he did as a teenager. Deconstructed. Gary was the coolest kid in school (or at least he thinks he was), and because of this, he hasn't moved away from his teenage persona. His hedonistic partaking of drugs and alcohol is an attempt to move away from the reality of his miserable existence, and his inability to drop his teenage pursuits as he approaches his forties is seen as more pathetic than charming. According to Word of God, Gary is meant to draw parallels with people who attend their 20th/25th high school reunions and find that they have accomplished very little with their lives. Reconstruction in that with his view of himself, he is much happier as a sword-wielding hero in the post-apocalyptic, pre-industrial world he creates by rejecting The Network.
X-Men: Days of Future Past: The younger Charles Xavier rejects all adult responsibilities after he succumbs to depression, and McCoy has to look after him.
X-Men: Apocalypse: At the age of 27, Quicksilver still isn't an independent adult because he continues to reside in his mother's basement, although his living space is a lot less cluttered than it was in Days of Future Past, which implies that his kleptomania has toned down in the past decade. He cracks a joke about his mother wanting him to get out of the house, and Peter acknowledges his Basement-Dweller status during the jet ride to Cairo.
You Can Count on Me features Mark Ruffalo as Terry, a drifter with no ambition who easily strikes up a relationship with his nephew by talking to him like an equal.
