Jasmine's father, the sultan, from Aladdin. At times he acts very childish, such as collecting toys and getting really excited when riding the carpet.
Humphrey from Alpha and Omega has signs of this. Matures out of it by the sequel.
"Baby Brent" in Cloudy with a Chance of Meatballs certainly comes across as one. He's the same age as Flint, doesn't seem to be doing anything with his life besides being popular, and still acts like a schoolyard bully. That he still reenacts the commercial he appeared in as a baby, diaper and all, seems to be a serious case of Lampshade Hanging.
And on the other hand there's Flint himself, who put fake security equipment in his lab and pretends to use it when he comes and goes. Even the kids in the neighborhood think he's weird.
Sid from Ice Age-so very, very much! Remember the time he got worried about what Santa Claus thinks?
Waffles, the horned toad from Rango, has a childlike personality and matching intelligence.
Prince John from Disney's Robin Hood. He's very whiny, often throws temper tantrums, constantly sucks his thumb and cries very easily.
Steve Stifler in the American Pie series. Especially in Reunion.
Similar to the Being There example below, 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 Bikini Bar and dance on stage with the strippers, etc.
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 she naturally freaks out.
When the two of them aren't killing gangsters, Connor and Murphy McManus, The Boondock Saints, are prone to bickering and tussling like ten-year-old boys.
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.
In another Burton flick, Charlie and the Chocolate Factory, is 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.
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.
Buddy from Elf is over 35 years old and still acts like a six-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 PitchJimmy 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's good with women, has a job as a well-respected junior high math teacher, and he has his own apartment.
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.
Jeff from Grandma's Boy: 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.
Marie Antoinette 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.
Johnny Boy in Mean Streets seems to be emotionally and mentally stuck at age 14.
The main characters of the film 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.
Pain and 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.
The protagonist in Teddy Bear is 38 years old and lives with his mom. They even share the bathroom.
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 fourteen years old!"
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. Reconstructed 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.
In The Wrestler, the main character is one, and it's shown in a tragic, negative light.
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.