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 12 year old gets real excited over a balloon? 10 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.
Deconstructed in Umineko no Naku Koro ni. The fact that 9-to-10 years old Maria acts younger than she is causes her to be bullied in school, and infuriates her mother to the point of abuse.
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. 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.
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.
A Fairly Odd Movie: Grow Up, Timmy Turner! has the titular Timmy acting like a ten year old, only because growing up means he has to give up his fairy godparents, whom he regards as his family (rightly so, given how neglectful his birth parents are). Ironically, due to his adventures with Cosmo and Wanda, Timmy has done or already knows how to do a lot of things in the adult world.
Fred: The Movie ages the titular 6-year-old Cloudcuckoolander to a teenager, but he acts the exact same as in the web series.
Ironically, Robin Williams in Jack (the 1996 film) played this.
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 twentysomething to stay with Batman / Bruce Wayne by bribing him with a cheeseburger.
A weird example: in Big, a 12-year-old kid is physically aged up to around thirty (and is played by Tom Hanks). His behavior in his adult body varies wildly, sometimes acting his age, sometimes acting around 7 or 8 (doing things like making vroom-vroom sounds with model planes in front of other people, something few 12-year-olds would be caught dead doing), and (later on) sometimes acting around 15 or 16.
In the V. C. Andrews novel Midnight Whispers, Jefferson is a nine year old who is treated somewhere between a toddler and a first grader.
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.
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.
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, 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.
As the show started to take a downfall, The Simpsons slowly started making Bart one of these (but not Lisa, whose Child Prodigy status is played Up to Eleven). This can mostly be attributed to the flat writing, so instead of making Bart precocious yet stupid at the same time, in the last couple seasons (worst offender is season 22) he is stupid and immature, with not a hint of his old self. Moms I'd Like to Forget is probably the worst offender.
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.