By the end of Code Geass the world's power is wielded by three teenage girls:
Tianzi, godhead-empress of China. She's just 13 years old and starts out as mix of puppet empress and Lonely Rich Kid, therefore her bodyguard Li Xingke's ultimate goal is to give her the influence and power she needs to truly rule. He succeeds with help of Lelouch.
Kaguya Sumeragi, head of the influential Sumeragi House, High Chairman of the UFN and Japan's representative on said council despite being vaguely 14 or thereabout.
Nunnally vi Brittannia, who rules Britannia as its 100th Empress.
Apparently the Europeans didn't want to be left out, because post-series supplemental material shows that they're also ruled by a teenage girl.
Actually, the trope best applies to the main protagonist of the series, Lelouch. Before he turned 19, he already had started the world's largest revolution and took over half of the world. After he lost that half due to betrayal by his subordinates, he then proceeded to take over the OTHER half, then went back and re-conquered the first half. As an eighteen year old, he was the first man to ever rule the entire world. He then proceeded to bring about the first era of true world peace by ordering his own death, at the hands of his equally young Dragon under the mask of Zero, thus leaving the path clear for Nunners becoming the Empress and with said dragon as a prospect Mysterious Protector.How's THAT?
Trinity Blood features two examples: His Holiness Alessandro XVIII, 399th Pope of Rome; and Seth Nightroad, Eternal Empress of the Methuselah Race. The former is respected by his citizens despite being a highly indecisive young boy; the latter is Really 700 Years Old but rules through a proxy so as to hide her child-like appearance.
Queen Mashiro of Mai-Otome is a Royal Brat at first, but becomes this after learning her lesson at the tender age of 14.
Integra in Hellsing started running the agency when she was still a young child. It helped that she also awakened Alucard.
Gaara from Naruto was elected as Kazekage of his village at some point during the timeskip, after which he is shown to be 15, in fact he is criticized (and even ridiculed) by some of the other kages for his age, inexperience and idealism. Its even mentioned that because of this, other leaders may refuse to take him seriously. Though in this case, Gaara is currently proving himself Wise Beyond Their Years as well giving the other kages an armor-busting inquiry and he's getting more respect now.
Tsunayoshi Sawada from Katekyo Hitman Reborn! is chosen to be Boss of the most powerful Mafia family in Italy at only 14 years old. Likewise, his fellow boss Uni, who is even younger.
Shangri-La features the Metal Age group, which is taken over by Kuniko at eighteen. She might have done it earlier had she not been in jail for two years before the series due to being a domestic terrorist.
Played with in The Twelve Kingdoms. Shushou, the Queen of Kyou, became queen at age 12 to save her ravished lands... 90 years before the story starts. And she still looks like a teenage girl, since she stopped aging after she ascended to the trone.
A bit straighter in Youko's case, as she's as much 17 and has just become the Queen of Kei.
In Kyo Kara Maoh, the protagonist, Yuuri Shibuya becomes the ruler of the Great Demon Kingdom at the age of fifteen.
And lets not forget Small Shimaron's illustrious boy king, Saralegui.
Emperor Hotohori or Saihitei of Fushigi Yuugi is 17 years old at the time the series begins and the ruler of the Konan empire.
Don't forget his son Reizeitei or Boushin, whom we see ruling when he's as much 10-12 years old.
One episode of Ghost in the Shell: Stand Alone Complex featured an improbable 12-year-old leader of a terrorist group, who went missing years ago after being captured by the same terrorists and seemingly hasn't aged a day. And then it turns out that she's not the original kidnapped girl, she's her daughter and the real girl has aged far more than she should have and now looks like her grandmother.
Kimba the White Lion has the title character inherit his father's kingdom while still a cub after his parents died.
In Zeta Gundam and Gundam ZZ, the official leader of Axis/Neo-Zeon is Mineva Lao Zabi, seven year old sole survivor of the Zabi family, although it's pretty clear to everyone that her regent Haman Karn is the one with all the real power. In Gundam Unicorn, Mineva is re-introduced to the story as a civilian teenager under the name of Audrey Burne, and it's still unknown if she'll fully take over her heritage.
In Gundam Wing, Relena Peacecraft first becomes the sovereign of the Sanc Kingdom at age 15, then later Queen of the World. Though she doesn't hold the latter position for very long, she wields very real power while she has it. And by the end of the story she's still working in politics.
Endless Waltz has Mariemaia, supposed daughter of Treize Kushrenada, leading her own army to conquer Earth in her preteens, although Dekim Barton is very clearly manipulating her actions and thoughts.
In Gundam SEED, Lacus Clyne (16) becomes the leader of her father's resistance movement following his death. By the end of Gundam SEED Destiny, two years later, she's the Chairwoman of the PLANT Supreme Council. Cagalli Yula Athha (same age) is also the chief representative of the nation of Orb following her father's death.
Jenny Sparks, leader of The Authority, died with the 20th century; her successor, Jenny Quantum, was soon born, and adopted by the team's Official Couple. Eight years later, after everything had gone to hell in a handbasket, little Jenny Quantum got sick of watching the adults screw it up: she aged herself to 14, reassembled the team and has led them ever since.
The Transformers fanfiction called A Child Shall Lead Them by Kenya Starflight about how Swoop the Dinobot becomes Prime instead of Hot Rod. Dinobots are canonically younger than the rest of the Autobots (at the time of the 1986 movie, twenty years old as opposed to millions of years), and the author's interpretation of the Dinobots gives them a younger mental age than the rest of the Autobots as well. For this reason Swoop (aka Pterodactus Prime) qualifies for this trope.
The Phantom Menace: Queen Amidala. In Attack of the Clones, Amidala has stepped down and Naboo has a new teeanged queen (and also mentions that she was not the youngest Queen in Naboo's history), showing that this is an enforced trope.
In the future of Idiocracy, the Secretary of Energy was in his early teens. Things like this happen when you give out Cabinet positions in a contest, though he didn't seem any worse at his job than anybody else.
Tropic Thunder. The heroin ring was run by a little kid, who was as ruthless as you'd expect a leader to be, and far more capable in combat than one would expect.
RoboCop 2, very similarly, had a preteen end up in charge of a Nuke ring.
Vanellope von Schweetz from Wreck-It Ralph, the nine-year old Princess and later President of Sugar Rush, a racing video game.
This is especially notable given that the Kings of Gondor and Arnor and their descendants (including Aragorn) live for centuries and most do not inherit the throne until they are over 100. When Aragorn finally does become King, he is 87 and still very young by Gondorian standards.
In Deltora Quest, the heir to Deltora turns out to be only sixteen years old.
Dunevon becomes king of the Copper Isles at about age four in Daughter of the Lioness, with his tyrannical aunt and uncle acting as his regents. They eventually have him assassinated and take the throne themselves. At the end, the rebels win and put Dovasary Balitang on the throne—she's thirteen. But she intends to do the actual job and has no regents or puppetmasters, and adulthood is considered to start at sixteen in this setting.
In Graceling, Bitterblue is about ten when she's crowned Queen of Monsea.
In Tanya Huff's Sing The Four Quarters, Lady Olinda sets up her nephew to be executed for treason so that she can control the new duc, his five-year-old son. The boy thwarts her in several ways before his father can reappear.
Gorath from The Riftwar Cycle, or more famously the RPG Betrayal at Krondor, becomes tribe chieftain at age twelve, and in subsequent years restores his tribe to strength and avenges their earlier near-massacre. Particularly notable because he's a dark elf, and would normally need to be at least a century old to be considered more than Just a Kid, let alone lead.
In Katharine Kurtz's Deryni series, Kelson Haldane inherited the throne at the age of thirteen, and was crowned the day after his fourteenth birthday. Also, King Javan Haldane, from the prequel series. Too bad he has an Evil Chancellor.
In A Song of Ice and Fire, there are several noble youths placed into positions of authority. Daenerys Targaryen, Robb Stark, Jon Snow, Bran Stark, Robert Arryn, Joffrey and Tommen Baratheon are some of the most notable examples. Before the start of the first novel, Renly Baratheon became the Lord of Storm's End when he was around 5 years old.
Michael Karl in Andre Norton's The Prince Commands. When Ulrich Karl, his alleged dead cousin, shows up so that Michael Karl is not the heir, he's an even better example.
All four of Fiona Patton's Historical FantasyBranion Realm series feature young royals and sovereigns as protagonists, often leading armies. In the first book, the ruler is 24, and much younger for large parts of the book; his mother took the throne at 25, and led a "Charge of Carnage" at 14. In the second, the ruler is five; her two immediate predecessors both took the throne at age 10. In the third, the ruler is 18, and was twelve when he cunningly poisoned an entire enemy army.
The Boy Emperor of the Agatean Empire in Mort, who sucessfully poisons his Evil Chancellor and former tutor with the dish the chancellor prepared for him ("I had a good teacher.") Apparently later killed by his uncle, the Royally Screwed Up emperor in Interesting Times.
Teppic is still a teenager in Pyramids (just graduated from the Assassins' Guild) when he ascends to the throne of Djelibeybi.
Coin from Sourcery was ten years old when he asserted his power, becoming de facto ruler of the world for a time.
Xuan at the end of Lords of the Bow is proclaimed Emperor of the Chin at the age of seven, after Zhi Zhong murders his father.
The Chronicles of Narnia primarily have the Pevensie siblings who lead Narnia's Golden Age starting when they were 13 (Peter), 12 (Susan), 10 (Edmund) and 8 (Lucy). Likewise, Caspian is 13 when he takes the throne.
At the end of The Obsidian Trilogy, we get the electoral version of this trope. After Arch-Mage Lycaelon Tavadon of Armethalieh resigned more or less in disgrase and what was left of the city's high council proved unable to agree on a successor from their ranks, a conclave of the all the mages in Armethalieh elected Cilarnen Volpiril Arch-Mage for both his valor in facing the demonic forces that nearly destroyed the city and his vast experience in dealing with the outlanders whose help was needed to restore Armethalieh's devistated lands. Cilarnen might have reached 19 at this point.
At the end of Deltora Quest series one, Lief of Del takes the throne. He's sixteen, and thinks more with his heart than with his brain... and yet, because the books were inspired by JRPGs of the mid-1990s, this ends well.
Not quite. He's still a child, but he did grow over the story to understand that there was a time and place for emotions to rule. He thinks with both his heart and his clever mind. And, by the end of the series,he's 18, and has learned even more about evil, temptation, and duty.
His father Leto Atreides became Duke at 14 after the death of his father Paulus in a bull-ring. He rules justly and fairly.
His first order of business? Banish his mother, who plotted to kill her husband.
And Paul's children technically inherit the throne shortly after being born, but their aunt Alia rules as regent until Leto II seizes control when he's about 12.
In Harry Harrison's Bill the Galactic Hero novel, the fleet admiral is an infant. His unintelligible utterings and cries are "translated" by his nannies into orders. No wonder the humans are having such a hard time beating foot-tall lizards. Then again, that entire universe runs on the Rule of Funny.
In Sergey Lukyanenko's Morning Nears, Marcus is a teenage bastard son of the ruler of the State with no claim to the throne. However, when it becomes clear that he may be the next messiah, everyone around him begins to think of him as their new ruler, which he will be once he gains enough supporters by making miracles. His father will be forced to abdicate.
In Edgar Rice Burroughs's The Mad King, the boy king had been imprisoned for ten years as he had lost his wits. His uncle, appointed Regent, said so.
After Emperor Justinian was blinded in a coup attempt a third of the way through The Belisarius Series, Empress Theodora avoided a succession crisis by adopting the eight year old son of her best friend (who, incidentally, also happened to be the stepson of the Roman Empire's best general) and putting him on the throne. Of course setting herself up Empress Regent and naming her husband Grand Justicar were not to be sneezed at.
Jenna is only ten years old when she is called to rule over the Castle in Septimus Heap.
Evading this is part of the background if the Codex Alera series, as Tavi's status as heir to the throne of First Lord is deliberately hidden for as long as possible to give him time to develop the skills he would need as ruler. By the time he learns his true name is Princeps Gaius Octavian, he's already had a full education at the most prestigious university in the nation, trained as a spy and assassin, and commanded a field army against impossible odds fighting an immense army of centuries old, two-meter tall wolfmen and won. By the time he's in a position to lead, he's managed to reach his mid-twenties.
The entire point of Ender’s Game by Orson Scott Card is to train Andrew "Ender" Wiggin (who is a six-year-old boy at the start of the novel) to command humanity's forces in a Bug War. Apparently, Ender's older brother Peter and sister Valentine are just as smart as him, but they don't appear to have the qualities the fleet requires. Peter has all the makings of a serial killer (he psychologically tortures Ender and Valentine and disects squirrels while they're alive), and Valentine has too much empathy for war. While Ender is at Battle School, Peter convinces Valentine to help him take over the world in order to prevent another world war, which is sure to start as soon as the buggers are defeated. They create online personalities called Locke (Peter) and Demosthenes (Valentine), which argue competings points with the eventual goal of having Locke having so much influence he can write policy for the entire world. By that point, the truth that they're really children will be known, but no one will care. And yes, this is exactly what happens. A short war breaks out after Ender beats the buggers (before he's 16), and Peter manages to stop it and is made Hegemon of Earth shortly after. Valentine's (or rather Demosthenes's) writings are still read and discussed thousands of years later. Oh, and Ender also manages to inadvertantly start a new religion before he's 20.
In Return to Neverend, David Inarai became the king in the land he and his mother created.
In Jack Vance's Planet of Adventure series: Traz. The nomad tribes' nominal leaders are generally teens or pre-teens—probably so the priests, who are the actual leaders, can have an easier time.
In the Honor Harrington spinoff novel Crown of Slaves, 17-year-old Berry Zilwicki, posing as Princess Ruth Winton of Manticore, becomes the leader of a group of former slaves. By the time the deception is revealed, no one cares. The full version of the trope is barely averted, as her coronation as Queen Berry of Torch takes place shortly after her 18th birthday.
In the Liavek series, the Levar (hereditary ruler of the city-state) is ten years old in the first book, with all real power held by her regent.
In The Psalms of Isaak, the Crimson Empress, ruler of The Empire of Y'Zir, is eventually revealed to be a toddler. Of course, though she's greatly revered by her people, she's not actually in charge of anything yet; Regent Eliz Xhum is the guy who actually runs the government. Oh, and the Empress's father, the immortal Wizard King Ahm Y'Zir, is still around too, but while he's revered as a living god by his followers, the fact that he's a senile, insane cyborg means he has little to do with actually running the empire he founded these days.
In The Windup Girl, 23rd century Thailand's current queen is a child - and that's about all we know about her, since she has a regent, or Somdet Chaopraya, ruling in her stead for the foreseeable future.
Several characters in Taylor Anderson's Destroyermen series, all female. Saan-Kakja, the Queen of the Fil-pin Islands, fairly quickly let her advisors know that she is the one who makes decisions on the isles despite her young age. Safir Maraan, the Orphan Queen of B'mbaado, also became a ruler of her city at a young age but remains not only a beloved queen but also a very capable warrior and general. Princess Rebecca Ann McDonald becomes the Governor-Empress of the Empire of New Britain in her early teens after her parents are murdered by a bomb planted by Lord-High Admiral McClain's people. One of her first acts as sovereign is to order the public execution of McClain, at the same time making radical changes to her nation's policies.
Though not as young as most examples, at 18, Egwene al'Vere in The Wheel of Time is by far the youngest ever Amyrlin Seat. Her predecessor had been the youngest at fifty, and many who gained the Seat were several hundred years of age. The women who raised her meant her as a figurehead, and referred to her as "the biddable child" in their planning, but she proved to be far more headstrong and savvy than any expected.
Bran Stark essentially becomes the Lord of Winterfell when Robb leaves his home to fight a war against the Lannisters.
After Robert Baratheon had won his rebellion against the Targaryens, he bestowed the title of Lord of Storm's End to his youngest brother Renly, who was only a child at the time.
Kid Nation is a reality show where a whole city is run entirely by children.
In the Bad Future of the first Legend of the Seeker season finale, the preteen son of Darken Rahl and Kahlan Amnell (who was forced to marry Rahl after Richard's apparent death), is a self-made orphan after executing his mother (for attempting to kill him) and confessing a guard to kill his father. He then proceeds to confess the entire population of D'Hara and beyond, ensuring ultimate loyalty.
Pair Of Kings the twin brothers Boomer and Brady are both crowned as kings of an island called Kinkow.
Sabaton's album Carolus Rex is predominantly about Charles XII of Sweden, in the Real Life examples below.
Religion and Mythology
The Bible provides the page quote. The Trope Namer is an unrelated but famous passage about the Messiah.
Judah had several child rulers, such as Joash, crowned at 7, and Josiah, crowned at 8. They were some of the few kings who "did what was right in the eyes of the Lord" Unfortunately, the former went off the rails in his adulthood.
By most sources, King Arthur was somewhere in the 15-to-18 range when he was crowned High King. The other rulers were not happy.
Elisabetta Barbados, the 13 year-old Holy Sacred Empress of Abel in Anima: Beyond Fantasy. In fact, she's known as the Child Empress.
Shakespeare's Henry VI (also a historical figure) was crowned king at nine months old, after Henry V died. This is an instance of a child king being portrayed as wholly negative, as he is too young and unknowledgeable to keep his courtiers from the petty in-fighting. Because he has no father to guide him, Henry stays weak and naive, which eventually leads to the War of the Roses.
"Henry VI, in infant bands crowned King/Of France and England, did this king succeed/Whose state so many had the managing/That he lost France, and made his England bleed." (epilogue, Henry V)
Antony And Cleopatra: Octavius Caesar, though older than most examples, is noted in play by various characters for only being in his twenties and ruling Rome.
Prince Roan from Grandia II. Who spends most of his time as a main character in the hero's party and who continues to fight with them even after circumstances have forced him to take the throne.
Final Fantasy XII had Larsa Solidor who was picked to be the next emperor by his dying father because his older brother Vayne was Obviously Evil. Once both his brother and father realized that the Evil Chancellors would try to manipulate him they decided to have Vayne murder dear old dad and frame the council in order to protect him. Which caused Larsa to not become the emperor until the very end of the game.
In Final Fantasy VIII, Balamb Garden trains students from the age of 5 to become mercenaries, with several students taking on positions of leadership by the time they reach their late teens. The results of this practice are VERY mixed. It eventually turns out that the reason for this practice is because Edea Kramer knows that Squall will, in his teens, defeat an incredibly dangerous Sorceress from the future, so they have to train these kids into leadership positions at a young age to keep the time loop stable.
Final Fantasy IX had Princess Garnet, who turned 16 at the start of the game and by the end, she was ruling her kingdom after the death of her mother. She seemed to handle the job quite competently, as it's implied she was able to rebuild her destroyed Alexandria in the span of only a few years.
Final Fantasy Tactics had Princess Ovelia, and Prince Orinas, each set up with a regent and sent to lead troops. Ovelia was not quite 18 at the time, while Orinas was 2.
Though the game kept an active calendar, and with a little grinding, you could age the boy prince into his teens, twenties, or even sixties, thus destroying the story.
Lymsleia is installed as a puppet ruler in Suikoden V. Unfortunately, even though the bad guys keep her aide hostage as leverage, they don't count on her actually exerting her power as Queen.
Mercedes from Odin Sphere takes up the throne after her mother, the fairy queen, dies from battle. She is quite overwhelmed by her responsibilities early on, but in time grows to be an extremely capable ruler.
Not really, while he was indeed the offical ruler everyone knew that he was more or less balantly controlled by his advisors... And Bolvar usually defered to Lady Katrana until it turns out that she is Onyxia in disguise and has charmed him (the magical way) to agree always with her opinion. It's a classic-era storyline
In Might & Magic VI, Prince Nicolai , who seems to be only ten years old or so at the start of the game, is the monarch of Enroth, technically, while King Roland is missing. (His advisor, Regent Wilbur Humphrey rules in all but name; Nicolai fells he's in a Gilded Cage where he is, and convinces the Player Characters to "kidnap" him to let him get out for a while, a major part of the plot.)
In first part of Virtual Villagers, the birth of the Golden Child signals a new era for the villagers, as he guides them to a new area on the island (continuing on the next game).
One of the rare villainous examples is Price Julius from Seisen no Keifu, who at the tender age of 15/16 is pretty much the one holding the power instead of his father, Emperor Arvis. He also directs the horrible child hunts and is the Dragon-in-Chief to Big Bad Manfroy. And then we have a straighter example in his half-brother and rival Seliph, who at the end of the game becomes a benevolent Emperor at the age of 18 - with the help of his somewhat younger girlfriend/wife (if he's paired up) and Julius's more benevolent twin sister, Princess Julia.
There's also Leif, who at age 16 is the leader of a resistence army in Thracia 776 and then joins Seliph's own group. If he makes it to the end, he becomes the King of Lester ( and later, of the New Kingdom of Thracia which includes Thracia and Lester) and rules peacefully alongside his older sister Altena (who is just 20) and his also-around-16 girlfriend/wife ( his childhood friend Nanna).
Fire Emblem Awakening provides a backstory example in that, while she's an adult now, Emmeryn of Ylisse initially became the Exalt before she was even ten years old, after the sudden death of her Knight Templar father in the war against Plegia. Technically she was a bit younger than even Sanaki when she started ruling.
Played more darkly in the Future Past DLC maps, which showcase the Bad Future. Chrom's daughter/Emmeryn and Lissa's niece Lucina (at most 18) is the Exalt now, but she and her friends are fighting a hopeless war against Grima. You and your team must help them win said war and help them earn their happy ending.
The mechanics of succession in Imperium Nova can cause this to happen. Most notable is Hogan Galle, who became head of house at age five, head of faith at age eight, and emperor of the galaxy at age fourteen.
Several shorts by Michael Ely, one of the game's writers, have Cha Dawn being carried into battle on shoulders.
Fon Master Ion, who is roughly fourteen, is head of the religion that governs the entire world. The predecessor he was cloned from also counts.
In the Medieval installments of the Total War games, it is entirely possible, through extremely nasty bad luck, to end up with a fourteen-year old king/sultan. This usually only happens if all other eligible faction heirs get killed and the king kicks the bucket, leaving only a young, untested son as the last available option. Alternately, the king could be batty and simply designate the boy as his heir over more eligible options because he's completely nuts. If the king is really nutty, the heir won't be a blood relation but a captain who was adopted as a son after he killed a single-unit company of brigands.
In Drawn: Dark Flight, your ultimate goal is to drive out the shadows over the kingdom so that the child Iris can take her place as rightful ruler.
Adulthood in Dwarf Fortress starts at twelve. Since children don't work, they tend to spend a lot of time socializing and making friends. Twelve-year-olds getting elected mayor (determined by friends and social skills) are not uncommon.
Lord Kalrathia from Dungeon Siege II is a Wise Beyond Their Years ruler of the town that bears his name, despite being in his early teens. At least until he is poisoned by one of his advisors who sympathizes with Valdis.
Happens often enough in Crusader Kings as rulers often die in battle or from a dagger in the back. In the first game this could be catastrophic, as children haven't developed their stats yet. In the second game a regent rules until they are sixteen and become adults.
Diablo III has Hakan II, emperor of Caldeum. He was found as an infant and declared by the leaders of the local religion to be the reincarnation of his predecessor. He is well-spoken for his age and seems to have at least some idea of his duty, despite spending his entire life in a palace. In fact, he's the demon lord Belial in disguise. It's unclear whether a living child ever really existed.
Welcome to Night Vale gives us Tamika Flynn, twelve when she let an uprising of children out of the feared Night Vale library, killing a librarian (one of the most feared creatures in the town) in the process. Thirteen now, she has been training an army of children with militaristic precision and the hand of her kill hung around her neck, and appears to be taking the reins against Strex-Corp as of "Missing."
In ThunderCats (2011), Thundera's King Lion-O, at seventeen is the youngest of the Blue Bloods in his group of ThunderCats. When Old Soldier Panthro joins, he dryly lampshades their lack of "adult supervision" prior to his arrival, while Lion-O's adoptive brother Tygra makes his age the subject of petty insults.
Tygra: You may be king, but I'm still older than you.
In the old ThunderCats, Lion-O was no older than WilyKat and WilyKit, but a faulty stasis pod allowed his body to age while he slept.
In the Young Justice episode "Coldhearted," the ten-year-old girl whose donor heart Wally delivers turns out to be Queen Perdita of Vlatava, and the only one keeping Count Vertigo from the throne.
Adventure Time: Princess Bubblegum for a few episodes when she was reverted into a thirteen-year-old.
France's Louis VII (age 11 as junior king, 17 as sole ruler), IX (age 12), XIII (age 9), XIV (age 4) and XV (age 5)note (as an aside, Louis XIV had a very long life; so much so that his immediate successor's grandfather, father, and older brother expired before he did). Also Charles VI (age 11), VIII (age 13), and IX (age 10). And to top it off, John I, who became king at birth... and died just five days later.
King Baldwin IV of the Crusader Kingdom of Jerusalem was only 13 when crowned and even then he proved to be an effective leader. At age 16, he gave the Crusaders a decisive victory in the Battle of Montgisar against Saladin, who barely managed to escape. Had he not been a victim ofleprosy, perhaps the crusader states would have lasted longer than they did.
England's Henry III (age 9), Edward III (age 14), Richard II (age 10), Henry VI (age 9 months), Edward V (age 12), and Edward VI (age 9).
Scotland's Mary I (Mary, Queen of Scots) and her son James VI, who would become James I of England. James got the Scottish throne after his mother was overthrown and was only thirteen months old. Mary's father died of what was probably cholera and she became Queen at six days old.
This was actually a pattern in the Scottish Royal House. Of James I through James V inclusive, the oldest at his accession was 15.
Margaret of Norway, was seen by many to be the queen of Scots from 1286 until her death in 1290. It should be noted that she died at age seven.
Joan of Arc, patron saint of France, was granted command of the French army at the tender age of 16. Under her leadership, the long string of English victories came to an abrupt halt. No wonder the English were so eager to see her removed when she was captured.
Egypt's King Tutankhamen, who became Pharaoh when around 8 and died at age 18.
Egypt's Pepi II Neferkare (reigned 2278?-2184 BC) came the throne at the age of 6 and died 94 years later.
The exact length of his reign is disputed, and was probably only 64 years.
Elagabalus, emperor of Rome, is perhaps the other side of this trope, doing everything with his power you would expect of a sex-crazed teenager with a short attention span. Hence, a Royal Brat. He was murdered by the Praetorian Guard 4 years after taking the throne at the age of 14.
He was subsequently succeeded by his cousin, 14-year old Alexander Severus. It didn't go well for him either; 12 years later, he also was killed by the Praetorians.
Charles XII of Sweden took the throne fairly at 14 and was just 18 years when he won the Battle of Narva (some historians claim his aides deserve more credit for that particular battle, no matter). He was more effective then many young rulers as he was a fairly good Strategist. However his recklessness and weakness in long-term planning kept him from rising to the level of a Wise Prince.
Mehmed II of the Ottoman Empire took Constantinople at the age of nineteen. His father Murad II initially abdicated in his favor when he was 12, and when dealing with an Hungarian invasion two years later Mehmed reportedly summoned the more experienced leader out of retirement thus:
"If you are the Sultan, come and lead your armies. If I am the Sultan, I hereby order you to come and lead my armies."
Joan I of Navarre, who became queen of Navarre and countess of Champagne at the age of three.
The future first emperor of China, Qin Shi Huangdi (then known as King Zheng of Qin), was only 13 when he became the king of the Qin state. After some 20 years of warfare, he carved out a huge empire that remains intact two thousand years later.
The last emperor of China, Puyi, ascended the throne when he was three. Child emperors were a favorite political tool for ambitious courtesans, as they were too young (and spoiled) to rule effectively. The early chapters of Romance of the Three Kingdoms chronicles the (fictionalized) account of such a succession of ridiculously young emperors. The Emperor Kangxi, however, ascended the throne at the tender age of eight, and his reign was widely considered to be the Qing dynasty's golden era.
Emperor Kangxi's father, Emperor Shunzhi was only five years old when he ascended the throne.
This was pretty common in medieval Muslim kingdoms. An atabeg (literally "father-prince" in Turkish, but more like godfather/stepfather) was sometimes appointed to teach and assist a minor on the throne. Eventually, atabeg became a hereditary title roughly equivalent to duke or somesuch. The most famous hereditary atabegs are the Zengids, the rulers of Mosul; they are directly responsible for the Second Crusade (thanks to their pressure on the Kingdom of Jerusalem) and indirectly responsible for the Third (Saladin was their trusted lieutenant who took over Egypt for them; he quickly exploited the fact that the atabeg was a minor (!) to consolidate control over Greater Syria as well.
In 1867, 14-going-to-15 Crown Prince Mutsuhito took over the reins of Imperial Japan after the death of his father. He would be known in history as the one who started the Empire's renovation (for some, a Magnificent Bastard; for others, a Smug Snake with good PR and effective counselors)... Emperor Meiji.
Actually, many Japanese emperors were like this, such as the eight-year-old Antoku from the Tale of the Heikei. It was practice for hundreds of years to keep very young men on the throne so that the real ruling of the country was left to the members of the Fujiwara clan, serving as regents, who also married their children into the line whenever possible. Many emperors abdicated early, and took on more power and authority as "retired emperors" while their younger successors were kept cloistered. And, of course, for about two hundred years during the Warring States period the real power fell to whomever was Shogun just then.
The children's crusades, both led by young teenage boys with great expectations. Suffice to say that they didn't turn out so well.
Russia's Ivan the Terrible (sixteen at the start of his reign), Mikhail Romanov (also sixteen), Peter the Great (ten) and Peter II (eleven, died at fourteen). The most extreme Russian example is Ivan VI: enthroned at the age of eight weeks, overthrown little more than a month later, imprisoned for the rest of his life, killed nearly a month before his 24th birthday.
The Sassanid Empire took this trope to the Logical Extreme when Shapur II became the only known monarch to be crowned in utero. In other words, while his mother was still pregnant with him, a coronation ceremony was held and the crown was placed on her belly, making him the next rulernote In most cases like this, they at least wait until the umbilical is cut and the kid washed up a little before breaking out the regalia.
A similar case occurred in Spain with Alfonso XIII, who became king at birth.
Here's a question: What if he'd been female? In Spain that wouldn't have been a problem (Alfonso's grandma had been the reigning queen Isabella II), but the rules did not allow that in Sassanid Persia.
It would have also been an issue in Spain at the time. Had he been born a female and the rightful ruler more than 200 years prior, it would have been fine..but when Alfonso's ancestor came to the throne from France as Philip V, he introduced France's Salic Law to Spain. Isabella II's father needed special and highly controversial legislation to allow her to become ruler in the first place, and it was a major reason for the revolution that eventually deposed her. Making it more of an issue is that while there was a powerful faction in Spanish politics at the time who wanted to make the monarchy into what it basically is today, it was far from certain that they would win.
There were two Sassanid empresses, Borandohkt and Azarmidokht, that ruled, albeit briefly, in the 7th century.
Jean-Claude "Baby Doc" Duvalier, son of François "Papa Doc" Duvalier, became President of Haiti at the age of 19.
Dom Pedro II became Emperor of Brazil when he was five years old. Nearly 60 years later, he became perhaps the most popular monarch ever to be overthrown. It's said that the only person in Brazil other than those responsible for the coup who didn't mind was Pedro himself, who'd grown tired of ruling. The citizens were pissed at how he was treated.
King Oyo of Toro is a modern-day example. Though he's technically an adult now, he was crowned at age 3 1/2.
William the Bastard not only had to deal with becoming the Duke of Normandy at 7 years old, as his name obviously implies, he was the bastard son of a tanner's daughter. As such, he dealt with multiple attempts on his line and his title, but he made it into his teens, was knighted by the King of France at 15, and by 19, was routinely seeing off all his rivals on the battlefield. Unlike most child rulers, William achieved great heights, the most notable of course being the defeat of King Harold Godwinson of England and taking the throne himself.
Sobhuza II of Swaziland ascended to the throne at the age of six months on the 10th of December, 1899, and reigned until his death 82 years, 254 days later, in 1982, making his reign the longest reign of recent history, and the third longest of all time.
King Carlos II (the Accursed) of Spain was not quite four when his aged father Felipe IV died at 60. That was the leastof his problems, and he died at 38 having never held effective power
Princess Wilhelmina Helena Pauline Maria of the Netherlands was not even born when her eldest half-brother Willem, Prince of Orange died and was only four when Spare To The Throne Alexander succumbed to illness (incidentally, her mother was younger than either of them). King Willem III passed in 1890, leaving the throne to her, when she was ten.
Between the end of the Thirty Years' War and the start of the Napoleonic Wars, the Prince-Bishop of Osnabrück was alternately appointed by the Catholic Archbishop-Elector of Cologne and the Protestant Duke of Brunswick-Lüneburg. The last ruler, Friedrich Augustnote (Later to be styled the Duke of York and Albany), was appointed in February 1764 by Duke Georg Wilhelm Friedrichnote (AKA King George III of Great Britain)... and was born in August 1763.
At the age of 10 Ptolemy XIII became co-ruler of Egypt with his 18-year old sister Cleopatra VII, as well as being formally married to her. It didn't last long; after less than six months she ousted him from power entirely, though he did make a brief comeback three years later, forcing Cleopatra to flee to Syria. The ensuing civil war was ended when Ptolemy drowned while attempting to cross the Nile, and Cleopatra appointed their younger brother Ptolemy XIV (13 years old at the time) as co-ruler (and husband) in his place, although the latter's rule was in name only even more so than his predecessor.
In 1885, King Alfonso XII of Spain died with no sons, but a pregnant wife. The throne was left vacant until the birth of the baby, which turned out to be a boy who was immediately proclaimed King Alfonso XIII. There was even a debate about whether his reign should be backdated to before his birth.