He'll fight off hordes of monsters with techniques the likes of which master swordsmen would never equal. He'll wield weapons spoken of only in the oldest of legends. He'll save the world and everyone in it from destruction five times over.
And he'll do all of this while not even being old enough to drive.
The Kid Hero is, as the name implies, a child. The exact age varies up and down the scale, from 8 to 17, but they're never an adult by whatever the local standards are. The "base age" seems to be 14 or thereabouts, though their numeric age may not be revealed.
Your standard Kid Hero has a strong streak of immaturity in them. They might well regard the adventure as a game, albeit one with slightly higher stakes. (Often their starting weapon is explicitly made of wood.) Expect this to be scrubbed out of them in short order, usually when the place they call home gets wiped off the map.
Kid Heroes DO have some things going for them, though: their idealism and youth. A villain trying to perform a Breaking Speech on a Kid Hero is in for a rude awakening when he gets it thrown back in his face. Kid Heroes, quite simply, have a very clear view of right or wrong. Some may call it naive, but it serves them well, as they are basically impossible to corrupt. They're also at that age where their self-image is intricately intertwined with their friends, so they often kick ass through The Power of Friendship.
A Kid Hero will commonly, but not universally, have two primary adversaries; his most dangerous opponent is usually an adult villain who serves as the hero's Arch-Enemy, while his second most dangerous opponent will usually be a younger villain, closer to his own age or slightly older, who serves as the hero's Evil Counterpart. These two are rarely affiliated, but if they do work together, the adult will almost certainly be the Big Bad with the young villain as an extremely reluctant and rebellious Dragon.
Most commonly seen in console and computer role-playing games, as it allows for character growth to feel natural and skips the question of why they aren't properly equipped for the job. Also appears in the vast majority of animated action-adventure, both Western and Eastern.
If a television show is specifically focused on this character, then the description will most likely be along the lines of "Saving the world, and doing homework!"
If particularly well executed, The Brigadier and others won't look like monsters or weaklings for sending a kid to do their heroics for them.
This is Older Than Feudalism; examples include the Biblical hero David, and several Greek and Hindu mythic heroes (e.g., Hercules, who killed two large snakes sent by Hera to kill him in his crib)
See also Child Soldiers, a much more realistic (and darker) concept. Compare Kid Samurai. Contrast Enfant Terrible, a common antagonist of such a character.
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Anime and Manga
Basically Shonen anime and manga in general. That and magical girls.
Kouji Kabuto from Mazinger Z was sixteen when began the series. His Battle Couple, Action Girl Sayaka Yumi and their friend Boss were the same age, and his brother Shiro was roughly ten-years-old. Tetsuya Tsurugi and Jun Hono from Great Mazinger were slightly older (although you would be hard-pressed to tell with Tetsuya, since he might be quite immature sometimes). It is hard to tell if Duke Fleed from UFO Robo Grendizer was underage or not, but Hikaru Makiba and his sister Maria were still teenagers.
Ryoma, Hayato, Musashi, Benkei and Michiru from Getter Robo were high-school students when the sereis began.
Hiroshi Shiba from Kotetsu Jeeg was also underage. And an amateur car racer. And Cyborg capable transforming into the head of an Humongous Mecha. And he looked after his mother and little sister after his father's murder.
Akira Hibiki from Raideen was a high-school student.
Kazuya Ryuuzaki from Daimos also seemed a teenager... even though he was an astronaut.
Deconstructed with Zambot3. The three main characters are pre-teenagers were pushed by their families to pilot a Humongous Mecha and fight a genocidal psychopath intended to wipe all humans off Earth. At the same time they were hated by the people they were intending protecting. Needless to say, they were through serious breakdowns, emotional abuse and several of them died before the survivor got a final mind-screw. This series went to extreme lengths to show children should not be used like soldiers, depicting infant abuse in a way unparalleled until then.
Mahou Sensei Negima!: Nagi Springfield during his youth certainly qualifies. At age 10, he wins the Mahora Tournament, and could fly without a staff and essentially teleport. Gathering a team of badasses and becoming a war hero, he receives the title of Thousand Master at age fifteen. His son and series protagonist, 10-year-old Negi aims to be just like him - he's already led his side to victory in the Battle of Mahora "game", and was able to go toe-to-toe with one of his father's contemporaries in an all-out fight.
Summed up in one line from Bleach; when Chad is offered the chance to end the battle and drink with Shunsui Kyōraku, he refuses. When asked why, he points out, "It's illegal for minors to drink;" it's quite easy to mistake Chad for an adult.. Along with Chad, Ichigo, Orihime, and Ishida are all 15. (17 now.) The shinigami most associated with them are Rukia and Renji who are biologically-speaking age-group peers of them, and Hitsugaya who, despite being a captain, is actually biologically younger than them all (around 13-14).
In the Galaxy Angel games, the player character is 21 and only one Angel is over 18. Galaxy Angel II has a 16-year-old player character and Angels from ages 11 to 21 (not counting Nano-Nano, who, as a Robot Girl of sorts, is probably about two).
Deconstructed in Neon Genesis Evangelion. 14-year old Shinji has a complete breakdown under the pressure of being the designated hero, whereas Asuka suffers a similar breakdown and Rei suicides all over the place. And Touji either gets mauled and crippled or dies, depending on which continuity you go.
One of the episodes is even titled "And he was aware he was just a child".
Shinji And Warhammer 40 Kun-Deconstructs the trope in spectacular fashion. That Shinji's mercenary force of Assault Terminators are recruited mostly from what used to be Macedonia does not go unnoticed by Admiral Yang.
Goku for most of Dragon Ball. Child Goku was so popular in Japan, that they tried to bring him back for Dragon Ball GT by having him wished into a child with the Dragon Balls.
In Dragon Ball Z, the role was filled by Gohan, and after the Cell saga, Trunks and Goten.
One of the main characters of GaoGaiGar, Mamoru Amami, is a rather interesting example of a normal boy who develops into one over the course of a series. Originally an ordinary grade school student, Mamoru becomes the Tag Along Kid to the heroes of the Gutsy Geoid Guard because he's the only one who can purify Zonder cores and turn them back into humans. Apart from flight and a rather potent shield which he can't consciously manifest (at first), he has no combat ability whatsoever, and his first attempt at purifying a Zonder on his own nearly gets him killed. At some point, however, the sheer awesomeness of his coworkers began to rub off on the little guy, and by the end of the tv series he'd racked up a few Crowning Moments of Awesome despite still not having any real combat ability, and goes up against Humongous Mecha-sized opponents on more than one occasion without getting squished. Then FINAL rolls around, and Mamoru comes back after an extended absence on Earth with several shiny new Levels In Badass; his healing powers have developed into a full-blown Holy Hand Grenade, his shield is capable of tanking direct hits from mecha-scale energy weapons, and he gets a Final Battle with his very own Evil Counterpart. And just to put the cherry on the kid's Awesome Cake, Mamoru's courage and refusal to surrender single-handedly tips the scales of the Final Battle by powering up the G-Stones of all of 3G's giant robots, giving them the energy for a chain of My Name Is Inigo Montoya moments. Not bad at all for an eight-year old.
The planned though sadly cancelled crossover between Betterman and GaoGaiGar would have had him finally promoted to piloting a mecha along with Kaidou.
In Naruto, the main characters start around 12, in part II they're 15 and more recently Naruto's age is revealed to be 16.
15-year-old Garrod Ran, an ex Street Urchin, becomes the pilot of the titular Gundam XReal Robot, with his also 15-year-old love interest Tiffa Addil following him in his adventures. His Team Dad and mentor as well as Tiffa's adoptive father, Jamil Neate, used to be one as well... but he actually had a complete breakdown after the war, which didn't let him pilot a mecha in more than 15 years.
Luffy from One Piece barely counts at age 17, but many people look at him as younger. Word of God wants to reinforce that minors still shouldn't drink. He's still treated as one after the Time Skip, even though he's now 19.
Practically every main protagonist in a Magical Girl series falls under this. At one time, the only ones that were of adult age are the 26-year-old Agnes Bell from My Wife is a Magical Girl and the 19-year-old Nanoha Takamachi from Magical Girl Lyrical Nanoha StrikerS. The latter did start her "career" at the age of nine and later is in command of several teen heroes.
The student meisters and Weapons in Soul Eater are mostly around the age of 13, which is either stated or can be deduced from various comments and flashbacks. Those who are noticeably older are the Thompsons (Liz, at least) and Tsubaki. Thunder and Fire are by far the youngest Weapons, or at least it appeared so until the latest chapter (briefly) 'aged' them via a power-up. There may have been some sort of timeskip following the Baba Yaga arc, but the art evolution and absence of a clear timeline make it difficult to tell.
In Monster Rancher, Genki is said to be around ten or eleven (The clearest indication comes from something a dragon says). It goes without saying that Genki is a complete and utter badass. His very first fight starts with him kicking a genetically modified velociraptor in the face. And not a little kick either, hell no. He is portrayed as being an uber, uber, powerful chosen-one who can do things nobody else can even the monsters, however.
Yusuke Urameshi from YuYu Hakusho, is 14 throughout most of the series. Kuwabara is around his same age.
Hunter × Hunter has Gon Freecss and Killuah Zoldyck, both of them were twelve when the series started. It's a deconstructed trope; on the one side, Killuah was tortured with poison and electro shocks since his birth due to his family assassin training, resulting he has become immune against them. When he is about to kill or threat someone, his personality becomes pretty cold and dark. On the other side, Gon started as a GokuExpy, but when the series progress, he has become more aggressive and deadly, even taking a innocent hostage to force his enemy to heal his mentor, only to learn that his mentor was already dead. This triggers Gon's ultimate Nen and he enforce himself to grow up into an adult, so that he can finally kill his enemy.
Every protagonist of all Digimon series, both anime and manga, except Digimon Savers (and even it still has a few).
The Bokurano kids thought they would become this. THEY WERE WRONG.
Played most obviously with Waku. He comes off as a stereotypical, boisterous Hot-Blooded shonen Kid Hero who is gleeful to ride the Humongous Mecha. He is the first to die, not even making it to the end of the first manga volume, and serves as the Decoy Protagonist.
Takeru Myojin/Mars from God Mars is stated to be seventeen years old.
In Shonen Onmyouji Masahiro, the grandson of Abe no Seimei, defeats hordes of demons from China, patrols the Japanese imperial capital for evil demons at night, and prevents the world from being over run by the armies of hell at the ripe young age of thirteen.
Ed from Fullmetal Alchemist who joins in the military at 12, and his brother Al who tags along at age 11
Almost all of the Solar Sailor Soldiers from Anime/Sailor Moon at the very least start out as "Hero kids"
The exception being Setsuna Meioh AKA Sailor Pluto whose already in her twenties when she appears
Sailor Uranus and Sailor Neptune technically start out in this group, being 16-17 when they appear, but both live like adults by the 5th season (not going to school, living on their own, and co-adopting Hotaru Tomoe along with Setsuna
The most extreme examples are Chibiusa Tsukino AKA Sailor Chibi Moon who in the Anime version ranges from the age 6 to about 8, Hotaru Tomoe AKA Sailor Saturn who starts out as 12 but after she is killed in Season S is reborn as a baby and ranges from physically an infant to about the size and stature of an 8 year old, and Chibi-Chibi Tsukino who's only 2 or 3 years old and can't even talk in complete sentences yet
Note: out of the three children listed above two don't really count in the Manga seeing as in the manga Chibiusa is 900 years old, she just never physically grew, and Chibi-Chibi is the future version of Sailor Moon, only a child for disguise
In the Yu-Gi-Oh! card game, there's actually a monster called "Hero Kid", which acts as an unofficial member of the "Elemental Hero" archetype.
Stan Lee took a wild shot by making Spider-Man one of the first true teen solo superheroes (i.e. non-sidekick; there was the Star-Spangled Kid back in WWII, but he didn't sell as well). The eventual widespread popularity of Spider-Man quite possibly is the root of the trope within the West.
The Ultimate Marvelversion of Spidey remained in the 15-16 age for the duration (160 issues) of his run. His successor, Miles Morales, is only 13. Just to hit Miles' age home, Nick Fury holds his hand when showing him around the Triskelion. Neither Miles nor Fury seem to be bothered by it. Both Spider-Men are realistically proportioned, so there tends to be David Versus Goliath dynamic with many villains.
Static, created by Dwayne McDuffie was intended to be inspired as a modern version of Spider-Man. Virgil Hawkins, the main character becomes Static after being affected by a mutagenic chemical at age 15.
Barring Katharsis and Tremor, none of The Movement's members are over 17. In fact, their leader, Virtue is only 16.
Years before, the prepubescent Billy Batson became Captain Marvel, and subsequently the most popular character in comic books. A lawsuit from DC eventually ended his reign, but the incident proved that children, likeso many demographics, react best to people like themselves.
Captain Marvel has the twist that he becomes an adult in his super powered form.
One retelling of Captain Marvel's origin deconstructed this somewhat. Big Bad Doctor Sivana got some intel that Billy Batson (homeless and living in a subway tunnel at the time) was Captain Marvel and sent assassins after him. Billy defeated them, but his best friend was fatally shot in the process. Billy was so enraged that he nearly violated Thou Shalt Not Kill with regards to Sivana. When Superman confronted him about it, Billy decided to show him his true nature.
Billy (after reverting back): My name's Billy Batson. But maybe it's too dangerous to be Billy anymore.
Needless to say, Superman was pissed and he called out Shazam for handing all of the responsibility and danger that comes with life as a superhero to a little boy.
Superman: He's a little boy. He should be worrying about homework and school and the crush he has on the girl sitting next to him in class! Not worrying about assassins killing his best friend! He's just a boy. This is a burden meant for men.
Though Shazam had pointed out that Billy was the only person on Earth who could be entrusted with the power of the gods. So he countered Superman's statements with "He's a boy . . . who could use some guidance." Cue Superman revealing his Clark Kent identity to Billy and the two becoming true friends.
Also pre-dating Spidey by some years was Superboy ("the adventures of Superman when he was a boy"), who debuted in More Fun Comics #101 in 1945, and soon moved to a lengthy run as the lead feature in Adventure Comics, plus gaining his own self-titled series (Superboy) in 1949.
The Marvel Universe superteam Power Pack all got their powers before the age of thirteen, and one of them had already retired from adventuring (more than once) by the time she was seventeen. Not to mention the fact that Katie, the youngest power sibling, was five.
Most of the cast of Runaways are teenage superheroes, though Molly Hayes probably epitomizes this trope best. She's 11 years old and has super-strength, plus invulnerability. However, like a young child, she tires easily and has to take naps when she's over-exerted. (In other words, she could probably go a round or two with the Hulk, but would probably fall asleep mid-battle.) Her idea for a superhero name? Princess Powerful (Hey, she's 11, give her a break!).
The Teen Titans, at least in their original run, was composed of the teenaged sidekicks of adult superheroes. Later versions of the Titans were mostly composed of college age superheroes, but still had members who were barely old enough to shave (like Beast Boy).
The Titans' successor Young Justice was also a group of Kid Heroes, even younger than the original Titans.
The Golden Age Star Spangled Kid and his modern age namesake are both teenage superheroes (with adult sidekicks). Courtney Whitmore, the modern Stargirl, still wears braces on her teeth. At 17 years old, she is the youngest member of the JLA.
Loki of The Mighty Thor was reincarnated as a kid. He skips most of the negative tropes because he's got a focused mission (save Thor from the Serpent), it's pointed out that he's in a lot of danger (he's a god, but a god in the body of a kid without his magic), and most of all, he's not acting like Scrappy Doo and charging into situations (at least physical situations) that he can't handle. His book is considered one of the best Marvel is putting out.
People forget that the X-Men started off as teen heroes in a school for gifted youngsters. After they grew up, various young mutants would join the team. Grant Morrison brought back the school concept during his run.
There is also New Warriors who started off having mostly teenage members, although, like the X-Men examples above, they all grew up.
Franklin and Valeria Richards, children of the Fantastic Four, have recently become full-fledged heroes and adventurers alongside their parents (during the same time period that brought us Kid Loki and Miles Morales - so perhaps as part of a wider comeback for younger superheroes).
Beth Lestrade in Children Of Time, fifteen in her first appearance and then a physical age of seventeen throughout the events of the "season finale". Justified in her being the daughter of a New Scotland Yard detective inspector.
All three children in The Incredibles. Interestingly, they start off with attitudes fairly realistic to children their age, just with superpowers. But by the end of the movie, the whole family is happy to take even the tiny baby with them on life-or-death fights against super-villains.
Star Wars: The Phantom Menace. Anakin Skywalker, who single-handedly won the Boonta Eve race with a home-made podracer and (accidentally) won the Battle of Naboo at the age of nine. In the EU, his grandkids are also examples. His children are not, however, with one wanting to go to the Imperial Academy when he starts his adventures and the other having already served as a Galactic Senator. Just like her mom at her age...
Harry Potter had his first adventure at the age of eleven and in the last book he defeated Lord Voldemort at seventeen. In addition to Harry himself, the series includes Ron Weasley, Hermione Granger, Neville Longbottom, Ginny Weasley, Luna Lovegood and the list goes on. On the other side, Draco Malfoy became an official Death Eater at sixteen.
Done somewhat differently in the Grey Griffins book series, in that while the main heroes are four children, adults frequently end up having to rescue them or help out, and are also the ones who do the overwhelming majority of the actual fighting. On the other hand, the kids tend to do much adventuring on their own, usually using their wits.
The younger a wizard, the more raw power they have available.
A wizard is first offered his/her power by the Powers That Be when s/he is the best (or the only) solution to a particular problem, a problem which was directly or indirectly caused by the Lone Power, the Big Bad of the series. Thus, whatever a wizard does after first getting his/her powers is always important, and often dangerous.
In Sandy Mitchell's Ciaphas Cain novel Cain's Last Stand, the students at the schola. Starting with Sprie who is piloting Cain's shuttle when the attack comes and analyzes it for Cain. At one point, Cain observes that the schola could pass for an Imperial Guard post if the faces weren't so young.
Most of the Oz books have a child as at least one of the protagonists. Dorothy Gale is the most famous, but the books also include Tip, Princess Ozma (when she takes part in the action), Betsy Bobbin, Ojo the Unlucky, and others.
All the half-bloods and Grover in Percy Jackson and the Olympians. It's debatable whether or not the Hunters of Artemis count too - technically they are immortal and therefore mostly in their thousands, but physically are prepubescent.
Grover's thirty-two in the last book, he just looks sixteen.
Thalia of the Hunter of Artemis has to count, because she was fifteen when she became a Hunter in the third book.
Robb Stark from A Song of Ice and Fire. The guy commands a good percentage of the continent's armed men while barely old enough to grow a beard.
The series is full of these. Jon Snow and Daenerys Targaryen are such examples, with the former becoming one of the youngest Lord Commanders in the Wall's history, and the latter becoming a queen and conqueror of slaver cities while just fifteen years old.
Arya is a deconstruction of the trope. Her coldly calculative personality is extremely disturbing to those she meets, as she has not even hit puberty yet and she's already a successful assassin.
Eragon in the Inheritance Cycle, though, in fairness, Eragon really isn't too far off from what his people consider the age of maturity when the series begins.
We All Fall Down by Robert Cormier has one of the few true subversions of this trope. The hero is introduced as an idealistic kid who calls himself "the Avenger" and daydreams of being a Vigilante Man, getting his chance after watching a break-in and rape. He's fully adult and completely insane.
Biggles is a borderline example, being eight months shy of his claimed seventeen years when he arrives as New Meat at the Air Training School in Biggles Learns To Fly. Most of the franchise is set some time after he reaches adulthood, however.
The six main characters of the Animorphs series start out at the age of 13, with the exception of Ax (though he's still basically the Andalite equivalent of a teenager, since he's an aristh, or cadet).
Septimus Heap: Septimus and Jenna are both 10 year old by the first book.
In Warrior Cats, several of the heroes are apprentices when we first see their points of view - the feline equivalent of 10-18 years old - and some PO Vs start even younger.
Viciously deconstructed in the Alex Rider series. The eponymous protagonist is bullied into a job as a spy that he doesn't want and the awful psychological toll on him is so bad that it's unlikely he will ever recover.
The protagonists of Roald Dahl's books are typically young boys around eight or nine years old (Charlie, George, James, etc.), In a few cases, young girls were used as main characters instead (Matilda, Sophie and the nameless heroine of The Magic Finger).
The protagonist of Strength & Justice is 15. So's his girlfriend, who occasionally shares the spotlight.
In the Rainbow Magic series, Rachel and Kirsty are young, though their exact age isn't specified.
In d20 Modern, the minimum age for starting occupations is 15 years old. In D20 Apocalypse, most starting occupations see their minimal ages being reduced, some to 12 years old. And upon reaching their 12th birthday, kids take their first level in one of the basic classes, and have no attributes adjustment. Cue 12 years old battling radroaches, raiders, scavengingpre-apocalypse ruins for spare ammo...
Crono from Chrono Trigger and Serge from Chrono Cross. Both still living at home with their mommies (and said home surprisingly does not blow up.)
Sora from the Kingdom Hearts series; he starts puberty between the first and second games, apparently immediately after the first one, if you go by his voice in Re: Chain of Memories. Although naturally that has to do with the fact that Haley Joel Osment only recorded it after Kingdom Hearts II.
Ryu from Breath of Fire III and Breath of Fire II's Ryu, who starts the game (along with Bow) as a young tyke literally equipped with a tree branch. His attacks even do a measly one damage to rub it in.
Most heroes — as well as most party members — of the Tales Series are in their mid-teens. Tales of Symphonia features two party members who're 12 physically. Tales of the Abyss takes it to new extremes with a major character who's only 7. Luke, because he's a clone.Tales of Vesperia stands out for having the party led by a 21 year old, the youngest human member is still 12 though (their is a 4 year old but he's a dog, so still an adult).
Several of the main characters from the Suikoden series of games.
Both Justin and Sue from Grandia - with a surprising dose of realism thrown in towards the end of the game, when Justin is forced to realize the seriousness of his quest, and Sue is forced to leave the party and return home after becoming ill, thanks to being unable to cope with the exhausting pace of the adventure.
As mentioned in the Anime & Manga section, any and every protagonist in Pokémon are teenagers or preteens when starting out; ranging from the 10 of Ash Ketchum to the (at most) 16 or 17 of the Pokémon Black and White protagonists. The only ones to be even in their late teens are Wes from Pokémon Colosseum (at eighteen) and an obscure side game protagonist (in her early twenties).
Pokémon-centric spinoffs such as Pokémon Mystery Dungeon and PokéPark Wii give Pikachu and various starter Pokémon the chance to become strong enough to defeat legendary opponents without ever having the opportunity to evolve first.
Secret of the Stars had its action divided between a group of warrior children tasked with saving the world, the Aquatallion, and a group of adults, the Kustera, whose only purpose is to provide support for them.
Perhaps taken to its extreme in Dragon Quest V, where the protagonist begins his heroic career at the age of five, and stores stock steel weapons and iron plate armor for children that age. In the final chapter of the game, the destined hero is not the main character, but his eight year old son. This includes legendary armor only the young boy can wear. Somehow it fits, despite having been worn by older heroes in chronologically earlier games.
In Illusion of Gaia, Will (the hero) appears to be approximately 12-15 years old, with everyone in the game referring to him as a "child". The game story spans over half a year, and naturally he does grow up quite a lot by the end of the game... but he was already fairly mature to begin with, unlike most kiddie heroes.
In Fallout 2, it is possible to design a character as young as 16. This has no bearing on the rest of the game, though, and the character is still very capable of performing the things other characters can. These things include drinking, doing drugs, having sex for various reasons, having a shotgun wedding, and becoming a pornstar (or fluffer, if lacking in talent). Oh, and killing your way through the game by various, interesting means.
Children of the Wasteland, a Game Mod for Fallout 3, goes to great lengths to alter the story so that the Lone Wanderer is chased out of Vault 101 at the tender age of eleven. It's surprisingly well-done, too, with all original interactions talking about the player's age being re-worked, a model pack to expand on the inventory available for children - which includes an exclusive shop in Megaton -, and even a milk crate item to allow access to interactions normally too tall for kids to reach.
Every main character of all three MOTHER games, except Duster, Boney and possibly Teddy. Exemplified by main protagonists Ninten, Ness and Lucas.
The heroes in Golden Sun are another example. Isaac, Garet, Jenna and Mia are 17 years old and Ivan only 15. They're referred to as "children" by most of Weyard's populace.
The Sonic the Hedgehog series is full of this, with Sonic himself being 15 years old. In fact, some of his pals haven't even reached 13 yet, including Tails and Amy Rose. Note that Tails is not only less than 13, he's 8, and one of the most competent heroes in the entire series.
Brian of Quest 64 is said to be 10 years old...or 5 years old according to the time period in which it took place.
The lords in the Fire Emblem saga are more often than not over 18, but some are young enough to be kid heroes:
On the same league, Celice from Geneology of the Holy War is 16 years old when he becomes the leader of La Résistance against The Empire. In addition, almost all of Celice's companions are around his age: his cousin Leaf (the hero in FE 5), his best friend Rana, etc..
Lyn's original age in the Japanese version of The Blazing Blade is 15. She was 17 going on 18 in the NA release, though. Eliwood and Hector are 17 in their own tales (16 in Lyn's tale, which took place a year earlier) as well. Wil is also 17, Serra and Priscilla are both 16, Rebecca and Erk are both 15, and Nino is 14.
Orta in Panzer Dragoon Orta is only an early- to mid-teenager. Iva Demilcol is even younger (not even ten) and he was recruited into the Imperial army.
Marco Van de Land from Battle Fantasia is one of the main characters, fitting the trope to a T. Following in his father's footsteps and wielding an oversized zweihander, he has a habit of crying and whining, much to the annoyance of the rest of the cast. His brother Urs may also qualify as a Kid Hero, but with different quirks.
Kid genius Commander Keen, of The Nineties video games of the same name. He even saves his own babysitter in one game, and it's discovered that the Big Bad who keeps trying to blow up the galaxy is Mortimer McMire, the kid who always bullies Keen at school because his IQ is one point lower — at 314! Oddly enough, the developer of the (good) Keen games was id Software, who went on to produce significantly more mature games like Wolfenstein 3D, Doom, and Quake.
Klonoa, the star of his game series, is stated to be twelve in Lunatea's Veil, therefore in its prequel he must be the same age or younger in Door to Phantomile—at least before it got remade, he looks older in the Wiimake of DtP. Though, since he's not human, the years might work differently for him.
Raz in Psychonauts is a prodigy at a summer camp for psychics. The adults have their competence either power dampened from the beginning, or Deus Exit Machina'd before anyone has a clue something is going wrong.
The Persona series makes frequent use of this, since each game frequently has a high school as one of its major settings, and high schoolers (usually in their 1st and 2nd years) make up most of the playable cast. Ken Amada is the most obvious example, still being in grade school.
In the Madou Monogatari series, Arle starts her adventures while in kindergarten.
The Wards in Worm are an organization of Kid Heroes. But remember that in Worm, Anyone Can Die still applies…
Ben 10 has a ten-year old protagonist named Ben Tennyson, who gains the power to turn into ten (or more) different aliens. However, he rarely acts more mature than usual for that age.
Although a bit more mature and determined in the sequel, Ben 10: Alien Force, 16-year-old Ben Tennyson still fits into this trope. Well, considering the English voice actor of this character who normally gets these kinds of roles in anime, video games, and western cartoons alike.
Aang on the Asian-influencedAvatar: The Last Airbender : that with tasks such as confronting a supreme monarch and restoring the political balance of the world, it seems almost cruel that such a burden should be placed upon a 12-year-old boy. To an end, this was essentially his initial reaction when presented with the entire concept, and most Avatars don't even start anything like this until they're 16. Fortunately, thanks to the series' focus on Character Development, he's moved forward a bit from being an irresponsible, blissfully carefree, stereotypical 12-year-old kid who uses the word "fun" in every other sentence. He does however occasionally get depressed by how much he is responsible for being the Avatar. Can't blame him though, because after all, he didn't have much of a choice in the matter.
Additionally, the task of undoing the effects of the hundred-year-long war rests on the shoulders of sixteen-year-old Firelord Zuko. Good luck with that!
Actually, whole main cast are children saving villages and world from psycho villains.
The sequel series stars Korra, a borderline example as she's seventeen at the start.
Dave, in the VeggieTales episode "Dave and the Giant Pickle." After all, it is a retelling of the story of David and Goliath.
The Teen Titans source material actually had the heroes as young adults overcoming the fact that they weren't kids anymore and continuing their heroic exploits regardless. However, since the cartoon series was being directed towards children, they were de-aged to around 15-17.
Young Justice with occasional deconstruction. The kids go through all sorts of hell, some deal with it better than others.
The Fairly OddParents: Ten year old Timmy Turner, while he often fits the trope Designated Hero because he causes as many problems as he solves, there have been cases where danger has occurred through no fault of his own, and he does step up to the plate and become a genuine hero. Wishology is the most known example of this.
Wakfu has Evangyline (17), Tristepin/Sadlygrove (16), and Amalia (14). But the real standout is Yugo, who is about 10-ish but defies some aspects of this trope by being more cheerful and positive, as well as *gasp* capable of handling Teleport Spam responsibly instead of just being a Bratty Half-Pint.
You think one Kid Hero, or a small group of Kid Heroes is something? The Kids Next Door (from, where else? Codename: Kids Next Door) is a worldwide organization full of them!
Adventure Time has Finn, Flame Princess, and several other borderline examples. Finn, the main protagonist and card-carrying hero, is a somewhat more realistic take on this trope, as he is genuinely nice and has good intentions but often screws up out of ignorance or impulsiveness. He doesn't always do the right thing, either, although he does when it counts. Although Finn ages in real time (he was 12 in the first season, and will be 15 as of episodes now produced but not aired before 3/14/13) and may well age out of the trope.
Truth In Television
The Royal Navy permitted newly-minted midshipmen to go to sea and at least nominally command enlisted men from the age of twelve in the era of Wooden Ships and Iron Men, and it was not unheard of for men to reach lieutenant before their 18th year. The minimum peacetime recruitment age for enlisted men in all three services remains a relatively tender 16 years to this day.
Many, many underage boys lied about their age to serve in the First World War, rather fewer in the Second. Some ended up promoted or decorated for courage under fire, though for most things ended rather worse.
Audie Murphy, of To Hell and Back fame was a mere 14 years old when he enlisted and was about as close to some of the wilder fictional examples of this trope as you're probably going to get in Real Life.