Acting Your Intellectual Age

A common trope among Child Prodigies and Brainy Babies: an intelligent youngster is essentially an adult in a child's body.

They have no interest in ever being a child. They eschew toys in favor of books and beginner scientific equipment. They immediately and eloquently denounce the logic of Santa Claus. They may insist that their parents read the classics to them as bedtime stories and thus scoff at the books meant for their age. Or they just read the classics — or, better, the encyclopedia — to themselves purely for fun. They would rather discuss world events than... play (How childish!). They may even already have a distinct, philosophical worldview, never mind that they haven't lived long enough to see enough of the world to form such a view. And so on.

One common effect of this 'adult-in-a-child's-body" phenomenon is that characters who act their intellectual age have no interest in kids who are their actual age. They may look down on kids their own age as savage or barbaric, and would rather associate with their intellectual peers, even if those peers are 4 or 5 times the character's age. Strangely, in the world of TV and media in general, neither party seems to care about the age difference, and the kid genius in question thinks nothing of it to discuss the latest political development with someone old enough to be their father/mother. Thus, of course, they are likely to be construed as Teacher's Pets in any academic setting. This aspect is only occasionally Truth in Television, as there are many cases of highly intelligent students deliberately dumbing themselves down in order to fit in.

Often, a character who acts their intellectual age is, supposedly, more sophisticated than the adults, even the educated ones, and will not hesitate to point out grammatical errors, logic flaws, or to criticize behavior. Often may grow into an Insufferable Genius, if they are not one already.

Note that this refers to kids who more or less behave like adults while still functioning in the world of kids, and having a normal childhood (or as close as a Child Prodigy can get to normal). If they are highly intelligent but thrust into an adult role, or otherwise traumatized into maturity, then they are Wise Beyond Their Years.

Somewhat Truth in Television, but reality is more complicated. Gifted children are usually all over the place in maturity, acting their intellectual age in some aspects, their actual age in others, and in between in other ways. And in some ways they may be different from non-gifted kids of any age. Emotional maturity is usually at age level or only slightly advanced. This makes finding friends tricky, since older kids see them as immature, while same-age kids don't understand them. (An amusing anecdote is a highly gifted 4 year old who wanted to leave a note for her same-age friend, before remembering with disappointment that her friend couldn't read!)

Contrast Innocent Prodigy.

Comic Books

 * Throughout his long career as a superhero, Billy "Captain Marvel" Batson has been all over the map with regard to this trope. Averting this trope is generally regarded as an important part of his characterization, and was certainly part of the character at his conception. Billy is a 12-year-old boy who becomes a super-hero in an adult body, and acts like it, reacting to the strange things he encounters with childlike enthusiasm. Various authors have forgotten this feature and played the trope straight, turning the adult Captain Marvel into someone with a personality indistinguishable from Superman. These runs are generally regarded very poorly by fans.
 * Amethyst, Princess of Gemworld is a more subtle case of averting the trope, and then playing it straight, than Captain Marvel. Like Marvel, she was a 12-year-old who gained powers and age when she travels to a magical kingdom. Played less childishly than Captain Marvel, Amy still had a tendency to be emotionally immature at certain moments, and the original creative team had to remind certain readers in the letters pages that it was because Amy was still emotionally a 12-year-old, whatever she looked like. And then the creative team was kicked off the book and the "Amy Winston" aspect of Amethyst's character was basically dropped.

Fan Works

 * Harry Potter and the Methods of Rationality: Harry James Potter-Evans-Verres.

Literature
"Aaron: Let's be absolutely clear. I know you're just someone dressed up. The Hogfather is a biological and temporal impossibility. I hope we understand one another. Death-as-Hogfather: Aaron: Correct. This is just a bit of seasonal frippery and, I may say, rampantly commercial. My mother's already bought my presents. I instructed her as to the right ones, of course. She often gets things wrong."
 * Charles Wallace from A Wrinkle in Time, which leads to him overestimating his capabilities, and gets him ensnared by IT.
 * Charles Wallace is a little different, though. He wishes to be accepted by his peers, but finds that he cannot hide his intelligence, and gets bullied for it.
 * Artemis Fowl...most of the time. Subverted Trope in the beginning of The Lost Colony, when among his greatest nemeses is the distraction that is puberty.
 * One of the first novels to feature a Child Prodigy was The Hampdenshire Wonder in 1911. The protagonist is not only a genius, but a truly superintelligent little kid who judges the whole human culture an "elementary, inchoate, disjunctive patchwork"... at age four and a half.
 * Wensleydale in Good Omens: His parents "called him 'Youngster'. They did this in the subconscious hope that he might take the hint; Wensleydale gave the impression of having been born with a mental age of forty seven." His favourite "comic" is Wonders of Science and Nature, and he insists on being the Only Sane Man in the face of Adam's ideas.
 * Discworld:
 * Nanny Ogg ruminates on the concept of people having "natural ages", levels of maturity they were designed for; her examples are herself, who is somewhere in her eighties but has always felt mentally nineteen or so, and some children who appear to have been "born" thirty-five, with their behavior more or less being textbook examples of this trope
 * Aaron Fidget in Hogfather:


 * Thomasina in Arcadia by Tom Stoppard. She functions as a child with regards to romance and sex, and an adult with regards to intellectual matters (Classics scholar, mathematical genius). Which serves as a reflection of the Central Theme of the conflict between Romanticism and the Enlightenment.
 * John Green's An Abundance of Katherines has Colin Singleton, who started reading at 3, loves anagrams, and creates a mathematical theorem to detail his relationship with all of his 19 girlfriends (all of whom are named Katherine).

Live-Action TV

 * River Tam from Firefly. She's shown, in flashbacks, to have been so smart that she could spot flaws in the textbooks Simon was studying from and uses surprisingly advanced terminology ("that whole section is fallacious"). Of course, she's also shown as being quite willing to be silly and engage in age-appropriate activities (like pretending she's an Alliance soldier who has gotten cut off from her squad when the Independents brought in dinosaurs).
 * Manny from Modern Family. In one particular episode, he has been having conversations with a grown woman online and arranges a date, neither suspecting an age difference.
 * "He's an old soul."
 * Micah in Heroes.
 * The old "Dakota Fanning" series of skits on Saturday Night Live was made of this trope. An actress portraying a fictionalized version of Fanning behaves this way.
 * In an episode of The WB show Everwood, Ephram talks about a young child and notes that she's mature "but without having that creepy Dakota Fanning '45-year-old-lady-trapped-in-a-10-year-old's-body' vibe."

Video Games

 * Van from Harvest Moon: Animal Parade prefers studying to playing, and always wants to know if your kid likes to read/go to school/do homework. He even teaches some of the other kids during events!

Web Original

 * The Whateley Universe has several examples. Ayla springs to mind as a self-acknowledged one, and Jobe is one who doesn't realize it. Ayla's reaction to realizing this is to try and bring his friends up to his level, so that they can at least understand why some things bug her so much. ("Why would I blow money on an expensive stereo system that's going to be outdated in three months' time, or on clothes that are going to be out of style in two weeks?")

Western Animation
"Young Barry: Interesting how we've developed such a sophisticated hierarchy at this young age. Young Steve: Shut up, Barry. Young Barry: (enthusiastic) I'm at the Bottom!"
 * Egghead, Jr. in the Looney Tunes Foghorn Leghorn cartoons. He was always reading books and otherwise acting in an intelligent manner, and didn't want to do the childish things Foghorn Leghorn wanted him to do.
 * Subverted with Polly from Stickin' Around. She plays off this trope accordingly, though it doesn't stop her from playing with the other kids.
 * Stewie in Family Guy tends to zig-zag this trope.
 * Dexter in Dexter's Laboratory adheres to the trope, except that his attitude toward girls remains entrenched in normal 9-year-old mode.
 * Lampshaded in an American Dad episode with a flashback of Steve's friends as toddlers: