You turn on the TV and start watching it. Apparently, some child actor is guest starring. But wait a minute. How old is his character? He says he's X, but he's acting like X minus 5. What kind of 14 year old gets real excited over a balloon? 13 year olds don't watch Nick Jr., hello! Lastly, why are a large amount of older teenagers buying trivial trading cards? To put it simply, that character is a Kiddie Kid.
Basically what happens when Most Writers Are Adults is applied, but instead of the writers writing about issues relevant to them, try to guess what kids act like and hit below the target age. On other occasions, writers may have a kid of the appropriate age in mind, but the director winds up casting a child in the role who's much older than intended. This may also happen when a show featuring teenagers is being marketed to younger kids, so they make the teenagers act like the target audience.
This can be a little Truth in Television for some, since not everybody acts as mature or shares the same interests as others.
Can be a side-effect of Competence Zone. Contrast Wise Beyond Their Years. Related to Menace Decay.
The titular character from Naruto is a strange case. In Part 1, he is just a little bit immature, but not enough to qualify for Kiddie Kid. Thing is, all of his peers are affected by Most Writers Are Adults, so the difference in maturity by his peers qualifies him for this trope. In Part 2, he grows up and acts his age, though.
The in-universe explanation is that he's acting out for attention, which may explain his lack of maturity.
Tony Tony Chopper from One Piece may count. He's 15, and only a couple years younger than Luffy, but he can also be very cowardly and is extremely gullible. Some people on the crew, like Zoro, defend him like a child caught in a battlefield when the situation calls for it. Slightly averted by the fact he really is competent on the battlefield, but his younger instincts get in the way. Part of it might have to do with that he was 8 years old when he ate the Human-Human fruit granting him his human-like capabilities. Usopp counts as well, he's 17 but hangs out with a group of 8 year olds and plays with them all the time.
Luffy himself is 17, now 19, years old and he certainly doesn't act like a teenager that age. He has the curiosity, love of adventures and new experiences, simple-minded logic, naivety and Brutal Honesty typical of a much younger child. Only in very rare situations when he gets pissed off or serious enough, he may show some leadership skills and actually act more mature than your average teenager. His childishness is complimented by how he looks like he has barely reached puberty yet...
Nana and Narumi from Kimikiss Pure Rouge are 16, but they look and act half their age. In one episode, they are shown to care about romantic love by having their ubiquitous plush frogs marry.
Zigzagged with Wendy (12 years old) in Fairy Tail. She is quite intelligent and regularly acts just as mature/like an Only Sane Man to the other protagonists which are 5-7 years older than her. But she seems to have a love for stuffed animals and toys that most 12-year-olds have outgrown. She has a rabbit backpack, has her room filled with teddy bears, and in one of the OVA's, she eagerly carries around a doll and wails like an infant when it is destroyed. She also sometimes shows Cheerful Child traits.
Molly from Runaways acts younger than her true age, intentionally, because it gets her attention, and because it makes her feel loved.
Also it gets adults to underestimate her. Which is useful, because if the emphasis is on the "cute" in Cute Bruiser than people aren't expecting it when the cute little girl tosses them across the block. Just ask Wolverine.
Fred: The Movie ages the titular 6-year-old Cloudcuckoolander to a teenager, but he acts the exact same as in the web series.
In Jack (1996) Robin Williams plays a 10 year old kid with a disorder that makes him age at 4x the rate. He however acts like he is 5.
Most children in the 2009 Astro Boy movie act a few years younger than their stated ages.
Robin in Batman Forever. The part was written for someone much younger than Chris O'Donnell, leading to jarring moments, such as Alfred convincing an obvious twenty-something to stay with Batman / Bruce Wayne by bribing him with a cheeseburger.
Susie in Too Much: The Robot With a Heart appears to be around 10 or 11, but acts very immature and childish for her age. The main plot is kicked off by her running away from home to be with her robot, who is now her best friend.
Possibly why Denny from The Room acts so weird—he appears to be in his late teens or very early twenties, and was played by a 26-year-old, but acts like a (strange) little kid who's just on the cusp of puberty and doesn't quite know how to handle it.
In the Confessions of Georgia Nicolson series, Georgia's younger sister Libby could qualify as this as she does not age along with Georgia though she should be about six by the end of the series. She still isn't toilet-trained, speaks only in toddler-gibberish, still uses a pushchair and her social skills only extend to shouting embarassing things at passersby and cheerfully torturing her male "fwends" at nursery school with make-up and garden implements.
Subverted (yes, an actual Subverted Trope) in the Harry Potter books. In the earlier books, Ginny Weasley was only one year younger than the Trio, but was treated and described as though she were several years younger. In later books, it's made clear that she had always had a stronger personality and was more shy due to both her child-like crush on Harry and being the Weasley family's only daughter among several boys, so she starts acting more outspoken as time passes. You can debate all day whether this was a Retcon or planned all along, but either way the subversion is there.
In the Diary of a Wimpy Kid books, Greg's friend Rowley is one of these — although he's in his early teens, he prefers to hang out with six-to-eight-year-olds, including holding a birthday party at the local Suck E. Cheese's and having several kids from karate class for a sleepover.
Played with in Good Luck Charlie with P.J. as a fan The Gurgles, a singing group for toddlers and TV show of the same name.
In the Hannah Montana TV series, Hannah's demographics are far more spread out than her real audience of young tween girls. This means older teenage boys would be obsessing over the Disney star on some episodes.
When they're not interested in romance, the Big Time Rush band can be rather goofy.
Screech from Saved by the Bell. He is the character with the least character development, and the goofiest to boot. It doesn't help that his actor is a few years younger than his costars, yet his character is the same age on the show.
Scott from Hip Hop Harry the actor portraying him is a teenager but he acts very childish, he doesn't know how to cross the street by himself, he doesn't know what a lot of things are, and he thinks it's a good idea to feed hot dogs and ice cream to a hamster.
Reese from Malcolm in the Middle, in the later seasons, especially with regards to school and authority figures in general. Despite being over a year older than Malcolm, in many contexts he seems like Malcolm's several-years-younger brother.
Ben from Friends, Ross' son from a prior marriage. The more he showed up in later seasons the more "the same age" he remained, despite the actor playing him was obviously getting close to being 12 rather than actually believing in Santa Clause. He was all but forgotten after Emma was born, and only received mentions in passing.
Doctor Who had characters like this in the early years:
Susan in Doctor Who suffered from this in some stories, such as getting far too excited about going paddling in the sea and getting a new dress in "The Keys of Marinus" and her general childish reactions in "The Daleks" (displaying disproportionate screaming, rolling-around-on-the-ground terror when the Daleks allow her to return to the TARDIS through a spooky-looking nuclear wasteland containing no living things and confirmed to be safe, and giggling uncontrollably at the Daleks's funny voices). Part of the reason her actress Carol Ann Ford left the role so early was because she was frustrated with her character being written this way, although it can be Justified by the fact that Susan isn't human, and is from a species that is considered a kid until they're hundreds. Her replacement companion, Vicki was written convincingly like a clever fourteen-year-old.
This was sometimes done intentionally in serials aimed at younger audiences in order to allow older characters to serve as Audience Surrogate characters. For instance, Steven (in his early 20s) and Dodo (a teenager) both act quite childish in "The Celestial Toymaker", doing things like laughing at clowns making the other jump, and getting cross with a creepy schoolboy character for cheating, being rude and not sharing sweets. It's a sharp contrast to their behaviour in "The Ark" and "The Savages", science fiction stories aimed at older children and teenagers, where they act their ages.
In Heavy Rain, Jason and to a lesser extent, Shaun, are incredibly stupid for their age. Near the beginning of the game, Jason, age 10, is alone with Ethan in the mall. Ethan immediately tells him not to wander off, only for him to ignore his father, wander around in a jam-packed mall, and leave the place to watch TV at a nearby store. When Ethan finally sees him across the street, Jason RUNS WITHOUT LOOKING FOR CARS AND GETS HIT AND KILLED. Two years later, when Shaun is the same age, one level is basically Ethan taking care of Shaun on a typical school afternoon. All he does ALL DAY is watch cartoons on TV with the same bored expression on his face. This includes Ethan making him a snack when at his age he could just look for something himself, sending him to bed at 8PM at the age where boys go to sleep between 9PM-10PM, and asking for his precious teddy bear. Shaun's case is a little Truth in Television since he comes from a broken family and coping with his brother's death, Shaun could complain to his mom and Ethan would lose custody if he didn't give as much care as Shaun expected, but he isn't completely excused.
The Nostalgia Critic admits to being bullied at school for acting younger than his age. This serves as foreshadowing, as later he realizes from a bad phone call to a director that he's not acting like a normal twenty eight year old man should.
Chris from Family Guy, Depending on the Writer. Some episodes will have Chris behave like a normal teenager, which has become more common in recent seasons, and others will make him easily entertained and somewhat shallow minded, as well as playing childish tricks anybody his age could see through, at age 15. Since his father is a Man Child, and since he is very similar to him, it may run in the family.
Jimmy Two-Shoes is a teenager, yet he's very excitable, gullible, and childish and seeks for fun wherever he goes.
Teenage Mutant Ninja Turtles? More like Tweenage Mutant Ninja Turtles. This is talking about the older Saturday morning cartoon, not the 4kids version, where they do grow up a bit.
Something is not right with Cleveland Jr. from The Cleveland Show. Sure, most people just view him as an optimistic nerd, but he also can't let go of some childhood memories. He still plays with his "Larry the Leopard" stuffed animal, and acts as though it's a real person, he often gets easily tricked by Rallo, who is 5 YEARS OLD, and he wants his dad to kiss him goodnight. This is explicitly stated as weird in-universe, and others his age typically avert this trope unless it's funny.
Butters from South Park can be seen as one, especially since the other children are as mature, if not more so than an average adult in the town. Supported by the fact that while most other kids are based on how they act when an adult isn't near, Butters lives up to the false stereotype that all kids are "little angels."
Bobby Hill of the later episodes of King of the Hill. Even though he is supposed to 14 years old, he occasionally acts very childish. This is mainly a case of Depending on the Writer; in some episodes he is fairly intelligent and mature and in others he is incredibly whiny, foolish, and immature. The 13th season has some of the worst offenders, with episodes like Master of Puppets and Reborn on the Fourth of July.
Dee Dee from Dexter's Laboratory is implied to be a sixth grader, making her at least 10, but acts more like someone half her age.
Macie turns into one in the As Told by Ginger episode "Family Therapy" after her parents start treating her like a four-year-old after forgetting her birthday. She starts to enjoy playing on a Rocket Tykes Swingset and a My First Jungle Gym, going to the local Suck E. Cheese's, has her thirteenth birthday party at a petting zoo (with kiddie-style decorations), and even starts dressing more like a four-year-old. Ginger notes that Macie is liable to get bullied if this continues. Macie snaps out of it after Ginger convinces her with her birthday present - a shirt that says "I'm a teenager, that's my problem".
Steve Smith from American Dad! is a Depending on the Writer version of this. While most of the time he acts like your stereotypical Hormone-Addled Teenager, some episodes give him childish mannerisms and interests such as toys and playing pretend. This could be justified by the fact that the show lacks a reoccurring "kid" character.
Cubert from Futurama is a 12-13 year old who looks and acts like he's around 9. A plot point of one episode was about how he felt the need to imitate everything he saw on his favourite TV show, even if it didn't make sense.