"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?
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 seven-year-old.
In The Wrestler, the main character is one, and it's shown in a tragic, negative light.
Johnny Boy in Mean Streets seems to be emotionally and mentally stuck at age 14.
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.
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 titular character in Jack due to accelerated aging. Many other Robin Williams characters qualify to varying extents.
In another Burton flick, Charlie and the Chocolate Factory, is Willy Wonka, a character eerily similar to Pee-Wee Herman. But, while Wonka appears to be completely unselfconscious of what a Man Child 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 Herman's world, Wonka's notices and is more than a little squicked by him.
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 male lead in Knocked Up also has many Man Child traits.
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."
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.
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.
TRON has 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!"
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.
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.
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.
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.
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.
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.