Ota Matsushita in AI: The Somnium Files is a 25-year-old otakuwho looks and acts like he's 15. He spends all his time and money on his e-girl best friend or maid cafes rather than looking for a job or even helping out with his parents' diner, which he justifies by saying he's working on his writing (although he lacks the confidence to actually publish anything even though he dropped out of college to do so). It's revealed in his route that his mother made him a Spoiled Brat, and his parents overworked themselves to support him after their diner was forced to shut down. His father suddenly died of a heart attack while his mother developed dementia that went untreated. At the end of his route and the Golden Ending, Ota Took a Level in Kindness after realizing what a burden he was and started working three jobs to afford treatment for his mother while still working on his writing on the side.
King Erik from Fantasy Life not only acts like a child but looks like one despite having a middle-aged wife and an at least teen-aged daughter. It turns out he's in a magic-based prank war with the equally man-childish Former Dark Sultan of Al Majiik and that one of the spells turned him into a child.
Fire Emblem: Three Houses has Edelgard who is normally fairly composed in public, but in private her favorite gifts are toys like stuffed animals and games, enjoying lieing about and gouging on sweets, makes mocking voice impressions of others, and handles her attraction to people more like a smitten little girl than anything else. This is most likely a result of her Dark and Troubled Past robbing her of her innocence early in life and affecting her emotional growth while leaving some desire to regain what she had lost.
Reno Jackson from Hearthstone: Heroes of Warcraft is a parody of every treasure hunter trope imaginable, combined with being not the sharpest tool, recklessness and irresponsibility, thus often causing trouble for his fellow Explorers. But there's a good reason for this, turns out he's a Blue Dragon who didn't realize he's one. So while he's an adult, by dragon standard he's a teenager in rebellious phase. Even his teammate even remarked This Explains So Much.
Yellow Heart, from Hyperdimension Neptunia Victory. Despite being physically powerful and having the visible age of the other CPUs (at least), she does everything to please her "parents", can only read and write very simple words, and is generally sharp as a sack of particularly dull hammers. Turns out she's an artificially altered Peashy, and therefore actually a child, despite "the big me" looking like an adult.
Marty "Kaboom" Moffat from the first game of Jagged Alliance, primarily due to brain damage he suffered from standing too close to too many explosions. His fondness for Roger "Dyno" Wittman (implied to be an Expy for Barney) is indicative of this.
Kaboom: I don't know...you gave my hero, Dyno, the boot! I used to watch his show every day! And two times on Saturdays!
Tingle from The Legend of Zelda series. A 35-year-old man who believes he is the reincarnation of a fairy and spends his spare time floating around on a balloon searching for other fairies. Ramped up in The Legend of Zelda: The Wind Waker, where his HQ is a tower where his cohorts spin the top around to make magic happen... or something.
In Littlewood, Dalton, one of the Hero's former companions, is a strange, spacey fighter who eats big, likes making loud noises, and who enjoys literally running around town. He's also afraid of vegetables. However, he's a positive portrayal, as he's shown to be a very caring and fun-loving guy whose energetic nature means he makes friends easily.
Mae from Night in the Woods is a 20-year-old college drop-out who acts like a rebellious teenager, compulsively committing petty crimes and having a hard time talking to other people her age about more adult concerns. This strains her relationships with her parents and most of her friends, except her partner in crimes, Gregg. Her flighty exterior also hidessome severe mental issues, including a textbook case of dissociation/derealization.
Most Hack and Slash Protagonists created and directed by Japanese developers like Hideki Kamiya and Hideki Itsuo, considering the trope consists of over the top flamboyant badasses that literally act like teenagers when dealing with hordes of demons/angels. They most certainly qualify.
Most of Capcom's most famous protagonists(with a few exceptions) are all manchildren/womanchildren with special abilities, quirks, and near emphasized cartoon quirks that highlight their games. Like Morrigan as an overstimulated sex demon with a teenage party girl mentality, or Date Masamune a brash and rambunctious feudal lord turned Japanese delinquent.
Justified in the case of Grunt from Mass Effect. Since he didn't technically HAVE a childhood (unless you count accelerated growth in a vat while a Mad Scientist plays context-free videos for you to be a "childhood"), he comes out of the tank nearly fully grown but without a great deal of comprehension of the world beyond "if you squeeze this thing here, the gun goes bang and kills something". As a result, while he's fairly smart, he still plays with action figures, pulls out Buffy Speak on a regular basis, and mostly views Shepard, the player character, as a Parental Substitute who will teach him how to do all the important krogan things like break stuff and set it on fire. In the Citadel DLC, after he sets a Citadel Security car on fire, steals it and crashes it, this exchange can happen:
Shepard: Grunt, apologize to the nice man for setting his car on fire. Grunt: (looking at his feet) I'm sorry I set your car on fire... [notices Shepard's expression] Grunt: ...and I won't do it again. [...and as soon as the police have left] Shepard: I love you, Grunt.
Big Boss, of all people, in Metal Gear Solid 3 and Metal Gear Solid: Peace Walker. He has a Phobia of vampires, he gets ridiculously excited about eating things, he makes childish noises of disappointment when told not to eat things, is rather fixated on his mother figure, thinks making cardboard tanks and using bananas as firearms is a pretty neat idea, gets along as equals with a literal child, and still believes in Father Christmas.
Otacon in his first appearance in Metal Gear Solid comes across this way. He's an intelligent but naive tender-hearted dork who likes dogs, robots, and anime and just wants to build cool stuff to protect people, and he is way out of his depth in a cast full of conspirators and killers.
Parsnip Bunner from Parsnip's behavior and understanding of things around him are particularly childish. However, despite his short stature and "soft" style of dress, Squirrel Girl refers to him as "old man", coupled with the fact that he's apparently old enough to own a house. He relentlessly pesters his neighbors without an understanding of boundaries, avoids the internet entirely out of a fear of cyberbullying, and handles knives in a, shall we say, playful manner, among many other things. LeRose outright refuses to lend him candles because she doesn't trust him with anything involving fire, and why he needs the candles in the first place is also the result of his behavior.
Parsnip, observing a painting of a bottle of wine: "Wow! A bottle of juice! It's like looking at a photograph!"
In Scary Robber Home Clash, the burglars are not only stupid but very immature. Both of them will jump on the trampoline in the yard while shouting "Whee!" Lester even more so as he talks about wanting to watch cartoons and at one point dresses up as a ghost to scare Felix.
Kamiko from Shadow Warrior 2. Despite seemingly being in her 20s, she acts very immature and complains all the time. turns out there's a good reason for this: she's half demon and literally only 3-years-old!
In Shovel Knight, King Knight is an example of this. This is made more apparent in his campaign, King of Cards.
Adding the "Childish" trait to a Sim in The Sims 3 will result in one of these. In addition to their Fun meter depleting more quickly, they'll be more prone to watching the children's television channel, can play tag and play with children's toys (options normally unavailable to older Sims), among other things. Adding the "Evil" trait alongside this one can result in the psychopathic version of this trope, too.
Sonic the Hedgehog games: Despite being 20-years-old, Vector the Crocodile is no more mature than the rest of Sonic's gang, often arguing with 6-year-old Charmy Bee.
F.A.N.G. from Street Fighter. He prances around and gleefully follows Bison's orders like a child and whenever his plan goes wrong he is prone to throwing temper tantrums. It's safe to say his other fellow Shadaloo lackeys have little patience for his shenanigans.
Mario easily fits into this category, though he's a distinctly positive portrayal of it. Aside from being a never-ending fountain of bubbly joy, he has absolutely zero qualms with playing with the Mini Marios his own company manufactures, indulges himself in any kind of fun activity regardless of how juvenile, and seems perpetually impossible to take the life-threatening obstacles he overcomes with anything but a whoop of excitement and charging headlong at it.
Mario might be full of youthful exuberance, but Luigi often just comes across as childish and immature. It is particularly evident in the Mario & Luigi series, where he's prone to crying like a child, though later games in the series tone it down somewhat. Superstar Saga and Partners in Time has loud, exaggerated sobbing, Paper Jam has him quietly crying into his hands. Even his voice comes across as more childish than Mario's, despite Mario having the more higher-pitched voice of the two.
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 6-year-old's.
Luke fon Fabre from Tales of the Abyss starts out very gullible, emotional, demanding, self-centered, petulant, frustrating and stubborn. Most of the time, he acts like he's 7-years-old. Which makes sense, because technically, he is. He was cloned from the real Luke fon Fabre seven years prior to the game's opening, and retained none of the original Luke's memories.
In Undertale, Papyrus, who is built like an adult man despite technically not being human and having an unclear age, has a few distinctly childish tendencies. In particular, he has his brother read bedtime stories to him, and is particularly fond of one titled Peek-a-Boo with Fluffy Bunny ("THE ENDING GETS ME EVERYTIME").
Max Hass from Wolfenstein: The New Order is an almost literal version, due (presumably) to the fact that a good 1/4 of his skull is missing. His room is filled with toys (some of which you can go out of your way to recover for him at the base), and he's got an acute fear of loud noises (namely buzz saws).
Subverted with regards to Pocket Circuit Fighter in 0 and Kiwami. While he is obsessed with the rather childish hobby of slot car racing, he is an otherwise mature and well-adjusted individual, and even expresses concern that his hobby might be hampering other aspects of his life. When called upon to take up adult responsibilities and step down from the Pocket Circuit Fighter role, he does so without complaint, his only issue being finding someone to take over... although he never abandons his love for racing. By the time of Like a Dragon, with some help from his son and Internet-based sales, he's made enough money from his tofu business to start a highly-successful go-kart circuit.
The Gondawara Family from the infamous Yakuza 2 side story "Be My Baby" are a bunch of yakuza who like to go to a special club, dress in diapers, and act like infants while women dressed as nurses coo over them. Kiryu gets dragged along but tries to back out respectfully, ultimately getting into a brawl with them when the Patriarch takes his reluctance as an insult. After the fight, Kiryu is thanked by the lower-level soldiers of the Gondawara Family, as their boss's fetishes were too weird even for them, but they didn't have a choice in the matter.
Ichiban Kasuga from Yakuza: Like a Dragon is a loveable goof who is obsessed with JRPGs, Dragon Quest in particular, and tries to structure his world view and even fighting style around their turn-based combat. Most of his friends humor him for the sake of reaching their objectives, but none can deny Ichiban's optimism and childlike hope are contagious and help him overcome some pretty severe obstacles.