Young Future Famous People

This is basically the Spinoff Babies trope applied to real people. The idea is to take Historical Domain Characters and depict them in their youth. It can be played for laughs--implying that people we normally think of as reserved, stoic, or even evil were once young and reckless themselves. (Or the opposite, where they are shown to possess the same traits and interests that later made them famous, even if realistically these would have come much later.) Of course, it may also be played straight.

If the person lived long enough ago that details of their adolescence or youth are obscure or even nonexistent, then this trope may be used as an excuse to ground an entirely fictional story in reality. Sometimes these characters are the leading characters, other times they may only serve as a fictional Cameo or a Shout-Out to a Historical In-Joke.

Obviously a standard trope in Historical Fiction. See also Historical Domain Character and In the Past Everyone Will Be Famous. Can overlap with Literary Agent Hypothesis, as, for example, when a minor character in a movie turns out to be the author of the book the movie is based on.

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Film

 * 1814 is a Russian movie about several key figures of the Golden Age of Russian Literature, such as Pushkin, Delwig and Kuchelbecker (as well as the Russian Iron Chancellor, Gorchakov, often considered Bismarck's archrival in international politics) when they were students at the Tsarskoye Selo Lyceum, aged from 14 to 18. The same applies to the first part of the Yuri Tynyanov novel Kyuchlya, which partly inspired it.
 * Jane Austen in Becoming Jane
 * John Keats in Bright Star. Though he was already a poet, technically he got the 'fame' part posthumously.
 * Coco Before Chanel
 * A Knight's Tale has Geoffrey Chaucer as a freelance Smart Guy with a gambling problem. Ends with an I Should Write a Book About This.
 * Max is a film about an art dealer in 1918 Germany who attempts to encourage a young fellow war veteran named Adolf Hitler to focus on becoming an artist. He ends up becoming more interested in politics.
 * The Motorcycle Diaries is about young Che Guevara.
 * The Pirates in An Adventure With Scientists includes a young Charles Darwin. He's on his way to being famous by the end of the movie, with a little help from the main character.
 * Nowhere Boy is about the teenage years of John Lennon.
 * Shakespeare in Love
 * Young Edison
 * Young Einstein
 * Young Mr. Lincoln
 * Young Mr. Pitt: A slight subversion, since William Pitt the Younger was twenty-four when he became Prime Minister for the first time.
 * The Young Victoria
 * Young Winston

Literature

 * This is the purpose of The Royal Diaries, a series of books that depicts the youthful days of several famous royal women.

Live Action TV

 * Leonardo
 * Hawking: features Stephen Hawking when he was an ambulatory, oddly cute 21 year old university student.

Video Games

 * Backyard Sports has many pros as kids, who are playable characters in the games. Originally just ordinary characters, but then became the Spotlight-Stealing Squad.

Anime and Manga

 * In Black Butler, a young eye specialist and insignificant writer named Arthur was invited to Ciel's dinner party, only to be caught up in a murder mystery. After he finds out, it is heavily implied Arthur does become a famous mystery writer, though his middle and last name are never revealed.
 * Afterschool Charisma is basically Clone High (and not quite so wacky). It's got teenage clone versions of Joan of Arc, Florence Nightingale, Marie Curie, Einstein, Sigmund Freud...

Comics

 * A pre-teen Aleister Crowley briefly appears in the graphic novel From Hell.

Film

 * Forrest Gump: Elvis, learning how to dance from a kid with leg braces.
 * John Carter: Edgar Rice Burroughs is John Carter's adoring nephew and heir.
 * Shakespeare in Love again, with John Webster making a couple of appearances as an adolescent boy fond of Gorn in plays.
 * Shanghai Knights:
 * A young Charlie Chaplin as a supporting character.
 * Ditto a young Arthur Conan Doyle
 * In the film Troy, Aeneas makes a cameo as the civilians escape through the tunnel. A charming little addition considering the movie is basically a big "eff-you" to accuracy of any sort.
 * The Tudors has the future Mary I, Elizabeth I, and Edward VI as supporting characters.
 * As does The King's Speech with the future Elizabeth II and Princess Margaret.

Literature

 * The 1632 series by Eric Flint:
 * Has a cameo appearance by an infant Baruch Spinoza, and a young-ish Oliver Cromwell who is thrown into prison by Charles I for a regicide he had yet to commit.
 * Young Rembrandt becomes a famous artist when people learn that in the future-that-was, he was a famous artist. This happens to quite a few people who haven't done anything yet, and most of 'em are driven crazy by the attention and expectations.
 * Rembrandt though does decide not to paint the paintings that are going to be famous, instead painting new ones.
 * An eleven year old Blaise Pascal ends up becoming a ward of Grantvile and becomes a frequent source of headaches as he attempts insanely dangerous experiments.
 * The cyberpunk short story Mozart In Mirrorshades featured a young Wolfgang Amadeus Mozart being introduced to synthesizers and electronic music by time-travelers from an alternate dimension. A young Marie Antoinette also makes an appearance.
 * In Neal Stephenson's The Baroque Cycle, in which In the Past Everyone Will Be Famous is heavily in action, it is a kid named Ben who brings Enoch Root to Daniel Waterhouse, after the former has arrived in Massachusetts. Then, cue the flashback to Enoch visiting Isaac Newton's school when Isaac was a kid...
 * Possibly the youngest Future Famous Person occurs in M.J. Trow's Lestrade novels. The inspector points at a baby in a pram and sarcastically declares the child could play Sherlock Holmes at least as well as William Gillette. It is then strongly hinted that this is a pre-verbal Basil Rathbone.
 * The Axis of Time series by John Birmingham features accidental time-travelers from Twenty Minutes Into the Future going back to World War Two, where their revelations about the future course of movies and music leads entrepreneurs to find young Elvis and Marilyn Monroe, among others, and present them with contracts to buy the rights to their future creative output.
 * In Time Cat, one of the adventures takes place in Italy, where the protagonists meet a young Leonardo Da Vinci and help discover his painting talent.
 * Played with in A World of Laughter a World of Tears set in an Alternate History so while some of the figures famous, they aren't famous for the things they are in the real timeline. For example, one viewpoint character, a young Bill Clinton will grow up for an important role for his work in civil rights but will never become President. Meanwhile another important viewpoint character, a young Jerry Brown, will.

Live Action Television

 * Boardwalk Empire features gangsters like Al Capone, Charlie 'Lucky' Luciano and Mayer Lansky in 1920 at the beginning of the Prohibition. They will not reach the height of their infamy for at least another decade.
 * The episode "The Girl in the Fireplace" from the new Doctor Who features young Madame de Pompadour.
 * A creepier version happens in Forever Knight. During a flashback, Lacroix is sitting on a train next to an unshaven German soldier returning from the western front after the cease-fire. He seriously considers turning him and comes within seconds of taking a bite as he is shaving, but something tells him that adding vampirism to the darkness he already sensed in his soul would be a Bad Idea... just as the soldier turns around to reveal that mustache. Which is a bit of an inaccuracy as photos of Hitler show that he had a handlebar moustache when he was younger.
 * This happened from time to time in Quantum Leap, due to Sam just having amazing luck. For example, on one occasion, he met a nerdy teenager named Stephen King, and on another brushed past a pre-teen saxophonist introduced as "Billy C from Hope, Arkansas".
 * Bigger list here.
 * The two-part Star Trek the Next Generation episode "Time's Arrow" featured a young, although the audience isn't aware of this until his last appearance in the episodes, before which he'd always been referred to by his first name only.
 * The TV miniseries Young Catherine featured a young Catherine The Great, played by Julia Ormond, during her rise to power in Russia.
 * The German television series Löwengrube (Lion Pit) bases on this, as it tells the history of the Munich middle class family Grandauer from the 1870s to the 1960s, following them through two world wars and the post-war episodes. One of the various examples would be the start of the first world war. In the police station (where the family patriarch works) a certain Austrian artist applies for German citizenship because he feels very German. Around the same time, the Grandauer's son (who is around 12 years old) is seen being buddies with a short and bespectacled dorky kid from his class, called.
 * The Russian miniseries Dostoevsky features Daddy's Daughters actress Liza Arzamasova as a young Sofia Kovalevskaya, who would later become Russia's first female mathematician.
 * This is a staple of The Young Indiana Jones Chronicles, though the titular character was not an example.
 * In an episode of The Dick Van Dyke Show Rob & Laura discover a hidden treasure inside a desk bequeathed to him by his Identical Grandfather: a picture of Grandpa as a baby. What makes it valuable (and this trope) is the young tall, beardless man who happens to have been standing on the stairway behind Gramps and his parents when the picture was taken.

Video Games

 * Assassin's Creed II has Leonardo Da Vinci in his younger years as an ally of protagonist Ezio.
 * Assassin's Creed Revelations has Suleiman The Magnificent as a 17-year old prince and grandson of Beyazid II,.

Web Comic

 * In The Dreamer, a young Alexander Hamilton is a supporting character in the series, as well as Nathan Hale, since he technically became famous posthumously. Benjamin Tallmadge also makes an appearance.

Western Animation
""Oh man, I have got to go back to law school.""
 * Clone High lives on this trope, but justified because they are all clones of the originals.
 * In Futurama, a flashback to Fry's childhood reveals that Mr. Panucci's delivery boy was a young law student named Barack. Young Fry mentions that he doesn't want to end up like that loser.

Other

 * A common Alternate History trope: expect to see a lot of people who only became famous in The Sixties in our history to appear as young soldiers in WW 2, for instance.

Real Life

 * On occasion a picture or video include (often as a bystander or generally not the main subject) a person who would achieve fame later. Here's Winston Churchill in 1918 in a Victory Parade in France. Churchill was already famous then. The Army officer in front of him is a then unknown Bernard Law Montgomery, a man who would achieve great fame in WW 2.
 * Here is a news-report about a Boys Nation visit to the JFK White House. One of the boys who shakes hands with the President? A teen-aged Bill Clinton.
 * A Cracked list of 10 commercials done by present day celebrities before they made it. Overlaps with Old Shame.
 * This is a picture of a young Lt. Colonel meeting the Secretary of Defense. Nothing strange about it, except the young Lt Colonel is Colin Powell. Moreover, the Secretary of Defense? Donald Rumsfeld, which makes it...very funny.
 * They're out there right now. Getting conceived, being born, in school, doing normal things. No one knows when or who they will be, but in 20 years, more or less, the newspapers, magazines, and the Internet will be throwing out their name left and right without even having to explain who they are - eventually someone's going to be making a TV Tropes page for them!
 * Books dealing with the Mexican-American war often mention minor things done by very junior officers that would not normally make it into descriptions of battle. This is because the officers would have names like Ulysses S. Grant and Robert E. Lee.