Really Was Born Yesterday - Television Tropes & Idioms

"She has a newness," he said. "Everything is for the first time... If I had learned she had been born this very morning, I would only be surprised that she was so old."

— Prince Lir, The Last Unicorn^note

Subverting the old joke, this character really was born yesterday, or this week at least, either artificially aged or just plain made the age they are now. They tend not to understand slang, or much of anything else, and will misunderstand social rules with usually comic results. May also be the result of a person being born normally but kept in some kind of stasis and never being conscious during their development since they still awake as a 'new' person.

Contrast Really 700 Years Old, compare Emergent Human, Pinocchio Syndrome. They're often prone to Blunt Metaphors Trauma as well. Can be coupled with Artificial Human if they were recently created. The extreme end of Younger Than They Look. Pairs well with Born as an Adult.

Note: Please be careful when adding robot examples as, broadly, they almost all count. As a general rule examples should be kept to those that act in a way that is very strongly reminiscent of the Trope description. Real Life examples may be added but should be related to medical conditions.

Examples

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Anime and Manga

Combat Cyborgs Otto, Deed, and Sette from Nanoha StrikerS, who, despite looking like teenagers, have barely a month of real memories (since they were artificially aged and imprinted mainly with combat skills).
Chii in Chobits. Not in a physical sense, her body is a robot that has existed for a while. But she's dead at the beginning of the series, and in the first episode she gets reincarnated into her own body. Her new self is very much born yesterday, having to learn everything.
In Trigun at least two of the characters are like this.
Yuki of Haruhi Suzumiya appears to be a high-schooler but is really a three-year-old alien interface thingy. Asakura is similar. They fit the trope even more closely later in the series, when Kyon and Mikuru meet them three years in the past (they could not have been more than a few weeks old at that time), and they still appear to be high-school age.

Comics

Young Justice: Superboy. His exact age depends on the continuity, but he's generally freshly cloned.
In Transmetropolitan, The Smiler's original pick for a vice presidential candidate was a two-year old first-term representative who had been cloned. He had a completely clean record, and no baggage from past scandals — that is, except the scandal of his actual identity.
Madelyne Pryor, Scott Summer's first wife in X-Men was eventually retconned into a clone of Jean, but despite this she's pretty well socially adjusted until she goes nuts after Scott abandons her.
Tesla, the title character in The Clockwork Girl, is only a day or so old when Huxley first takes her out to see the natural world. She's delighted with all the things she's seeing for the very first time. He responds that he thinks he's never really seen them before now, either.

Film

The Snowman
Frosty the Snowman
Pinocchio, a boy is carved from wood and brought to life.

Literature

The Bible: Adam, then Eve.
F. Scott Fitzgerald's ''The Curious Case of Benjamin Button", and the film of the same title
The Adventures of Pinocchio, the original source.
The Last Unicorn: Lady Amalthea is immediately noticeable as having a "newness" to her, where each movement she makes indicates that she's doing many things for the first time. This happens to be because up until that point, she was a unicorn.
Elva from Christopher Paolini's Inheritance Cycle aged physically to around five years old within a few days of her birth because Eragon, in his attempt to bless her newborn self, bungled a syllable and accidentally cursed her with the ability to "be a shield from harm" (instead of "being shielded from harm") . What's even creepier is that while she aged physically to around five in that time, she mentally developed the intelligence, reasoning capacity, and cynicism of a woman with several decades under her belt.
A Series of Unfortunate Events mentioned the idiom in one of the books, telling how you (the reader) were probably not born yesterday. The author adds, "Unless, of course, you were born yesterday, and in that case, welcome to the world, little baby, and congratulations on having learned how to read so early in life."
Thallo, from Paul of Dune is only nine, but appears seventeen or eighteen years old, his physical growth having been accelerated by his creators.
Gaia, from the Gone series, was born four months after conception as a fully formed infant, and looked about sixteen years old after birth.

Live Action TV

Various characters in Doctor Who.
Luke and Sky Smith from the Doctor Who spinoff The Sarah Jane Adventures.
Liam Kincaid, the protagonist of Earth: Final Conflict for seasons 2-3. Born with three parents (one of them an alien) he grows from infant to adult over less than one full episode.
The classic Outer Limits TOS episode "Demon with a Glass Hand" plays with this trope. Trent, the protagonist, begins the story by musing, "I was born ten days ago. A full grown man, born ten days ago." Actually, he's a robot from 1000 years in the future built to protect humanity from genocidal invaders.
Dawn Summers in the fifth season of Buffy the Vampire Slayer.
Kyle XY, the title character spent the first 17 years of his life gestating in an incubation chamber.
Adria, the humanoid Crystal Dragon Jesus of the Ori in Stargate SG-1, is carried for 9 months and born like any other baby, but she ages to young adulthood in about 24 hours.

Tabletop Games

A common occurrence in In Nomine, where the angel or demon who was first sent to Earth this morning will nonetheless usually be wearing a body that looks like an adult human.
- Although they aren't usually sent to Earth immediately after their creation, still there's a major learning curve for those new to Earth no matter how old they actually are, and it has much the same effect.

Theatre

In The Wiz, the Scarecrow's I Am Song is called "I Was Born on the Day Before Yesterday".

Rocky in "The Rocky Horror Show" (and its film adaptation) is actually born onstage during the song "I Can Make You a Man". He's a real hunk of a man-baby, and (naturally) has the mind of a toddler, which makes it kind of concerning that his first sexual experience happens when he's less than a day old.

Video Games

The Sylvari from Guild Wars: Eye of the North and Guild Wars 2 are this, as they are a new race just getting their foothold. You actually get to see the 'birth' of the first Sylvari in a cutscene in Eye of the North.
Grunt from Mass Effect 2, and he's basically born in your cargo bay.
KOS-MOS from Xenosaga is activated for the first time during the events of the first game. Although she does share data with a buggy prototype that was activated once before.
Rika from Phantasy Star IV is one year old, but looks just a little younger than Kyra, who is 18
Klaymen himself in The Neverhood. The world is a kind of Eden (both metaphorically and, er, metaphorically), so it's not surprising there would be some literally new people in it.

Web Comics

In Far Out, this is suggested to the main character as a possibility, instead of amnesia. Given he's a robot, it's a real one.
In The Inexplicable Adventures of Bob! this is the reason the three peanut butter monsters—Molly, Golly, and Jolly—act so immature. They're only a matter of months old. The only reason they don't act like infants is that they're hyper-intelligent and learn quickly.

Web Original

Belphoebe in the Whateley Universe is a cloned drow created by one student (Belphegor) using the cloning chamber he'd stolen from another (Jobe Wilkins) and accidentally imprinted with a copy of his own memories. Hence while chronologically she's 0, she looks 16 and is considered that age by the administration, staff and students.

Western Animation

Peep in Peep And The Big Wide World is a baby male chicken about as intelligent as a human first grader. In one episode he recalls his entire life, which actually did start yesterday.
The Last Unicorn: Lady Amalthea is immediately noticeable as having a "newness" to her, where each movement she makes indicates that she's doing many things for the first time. This happens to be because up until that point, she was a unicorn.
Young Justice: Superboy. Not quite "yesterday" but he's 16 weeks old in the first episode.
Ben 10: Ultimate Alien: Eunice is a 16-year old girl is found on a spaceship, later in the episode is discovered to be a Unitrix splicing Gwen's DNA.
The Canadian animated short To Be involves a teleporter that works by duplicating the "passenger" and killing the original. The film is about the inventor getting called out on the immorality of this. After the protagonist is responsible for one iteration of the inventor getting killed, she atones for the crime by teleporting herself, reasoning that the guilty her has been executed, and the "new her" is innocent.

Real Life

In very rare cases, a person's brain can lose track of its entire memory making it seem like it has been 'wiped', such as in this example. Victims generally retain a few links and don't lose everything, though it can happen (there are also very rare conditions where every memory can vanish, just like that, and with no warning at all).