Characters assume another character's age, often based on common assumptions of traits that character has, or there can be other reasons.
A common one is an aunt (or uncle). In many cultures aunts are thought of as old, so when the aunt turns out to be only a few years older than the niece/nephew, it can be quite a surprise.
- The Grape Nuts cereal television ad: "Mrs. Burke, I thought you were Dale!"
- Later followed by the Palmolive "Which hands belong to the daughter?"
- One Oil of Olay commercial had a stage psychic guess a woman's age, and undershoot because she was using the product. She plays along, and says the psychic was correct.
- It's a running gag in the Baccano! light novels that, despite nearly half the cast being immortal, only Firo (already a bit young-looking before immortality) has to deal with the annoyances of not looking his age, such as getting carded at age ninety, being detained for claiming he was seventy on his passport, and constantly being called 'babyface'.
- In Den-noh Coil Haraken's aunt, who is referred to offhand as being the commander of the Satchiis, is assumed by the main characters to be at the very least middle-aged. When she finally appears, it turns out that she's actually on the nearer side of twenty.
- This is actually the Berserk Button of Maria from Hayate the Combat Butler. She continually states that she's 'only 17' while everyone around her assumes she's older because of the aura she puts up, and being Nagi's Cool Big Sis/Mother figure.
- Hinata and Nishinoya in Haikyuu!! are often mistaken for younger than they are due to their height. In one instance, a volleyball referee even questions whether they're in middle- or elementary school. Both were in high school at the time with Nishinoya being a second-year. On the other hand, Asahi (a third-year in high school and 17 at the time) looks older and is frequently mistaken for an adult who was held back several years.
- The protagonist of the first story in Himitsu Kichi mistakes a sixth grader for a college aged student. In turn he mistakes her for a kid his age when she's 16 years old.
- In Lucky Star, Miyuki's mother looks so youthful and cute that she's actually often mistaken for her older sister rather than her mother.
- In Massugu ni Ikou, Mametarou mistakes Shokora for a puppy. Shokora is actually seven years old.
- The Monstrous Duke's Adopted Daughter: Characters frequently mistake twelve year-old Leslie for being about nine or ten, as she is smaller and skinnier than she should be for her age due to her abusive upbringing including not feeding her properly. This is exaggerated when she meets Bethrion. Between his only frame of reference being his younger brother and the fact that his family are all naturally large people, he initially assumes Leslie is five.
- In Uzaki-chan Wants to Hang Out!, this happens when the 19-year-old Hana Uzaki meets her friend Shinichi Sakurai's boss for the first time. He initially assumes Hana's a middle school student, much to her indignation, and disbelieves he misread her. A quick look down does convince him she's an adult, though.
- In the Fantastic Four, the Thing frequently mentioned his 'Aunt Petunia', with the implication that she was an old woman. When she finally appeared in the comic during John Byrne's run, Petunia (she prefers 'Penny') was revealed to be his uncle's second wife and an attractive woman about the Thing's own age.
- A flashback had a then-teenage Batgirl meeting Green Arrow for the first time. He casually flirts with her and cradles her. Barbara begins laughing and says his cologne smells like her father, which causes a squicked Oliver to back off. In hindsight Barbara says her costume made her look at least twenty.
- In the Splatoon fic Her Fractured Spirit, Callie is older than Marie. Everyone thinks it's the other way around because Marie is so mature and aloof while Callie is so free-spirited and exuberant.
- In My Master Ed, Van Hohenheim is shocked when he finds out Edward is sixteen, having assumed he was closer to twelve. Edward's not doing any better upon finding out Van's fifteen, mostly because Van's already taller than him despite being the younger of them.
- In Love Worth Waiting For, Mulan isn't able to figure out that Elsa is 23 by looking at her. Elsa looks younger.
- In RWBY ABRG, Weiss tells the teenaged Penny that she looks like a twelve year old.
- Mob in the Mob Psycho 100 gender swap fanfic Shigeko Kageyama AKA Mob because of the fact that as a female Mob carries herself more like an adult than most teenage girls, how quiet she is, and how she is always seen with Reigen, a grown man. Truth in fiction as females are often assumed to be older than they are once puberty hits.
- In The Loud House fanfiction The Nightmare House, Lisa, a preschooler, has a nightmare involving being sent back to daycare and put in a diaper. A teddy bear thinks she's a baby.
- In Anchors Aweigh, two Navy sailors take a runaway kid back to his home, where he lives with his aunt. They think she must be old, and are surprised that she is about their age.
- Another younger than expected aunt in A Few Good Men.
- In the The Baby-Sitters Club movie, the parent of one of Stacey's baby-sitting charges warns her in advance that her "little cousin" from abroad will be home when she comes by to baby-sit. She brings with her a little American flag to give him, expecting a young child. Instead, he's an extremely attractive 18 year old.
- In Indiana Jones and the Temple of Doom, Willie Scott dresses up beautifully for the dinner at Pankot Palace in hopes to impress the wealthy Maharajah. She's chagrined when he's revealed to be a kid around the same age as Short Round.
- Used with Sugary Malice from the evil fiance in The Parent Trap (1998) - where upon meeting Annie (disguised as Hallie), Meredith says she excepted a little girl as opposed to the "so grown up" eleven-year-old.
- Early in Aunt Dimity: Detective, Lilian Bunting asks Lori to entertain her nephew "Nicky" while she and her husband attend Prunella Hooper's inquest. Lori expects to receive a visit from a small boy, and is surprised when thirty-something Nicholas Fox introduces himself.
- In Mary Renault's The Last of the Wine, the main character's lover has a spazz attack when he realizes that the woman the main character has been calling "Mom" for the whole book is a stepmother and about the older lover's age.
- In Tailchaser's Song, Tailchaser is asked if he's 6 months old yet. He sheepishly mentions he's turning 9 months soon. In human terms, this is essentially equivalent to mistaking a 17 year old for a 12 year old.
- In the prequel Tales of Dunk and Egg novella "The Sworn Sword", Dunk is sent to meet Lady Rohanne Webber, who he knows to be widowed multiple times (and rumored to have murdered her husbands) and who has a terrible reputation for cruelty. As a prank, Dunk is introduced to one of Lady Webber's attendants and is told that she is Lady Webber, and he believes it, because the elderly and unpleasant looking woman jibes with his assumptions about what Lady Webber would look like. He then learns that a pretty young redheaded woman practicing archery nearby is Lady Webber, and that she is not nearly as bad as her reputation suggests.
- Used tragically in the first arc of Warrior Cats. The other Clans notice that the ShadowClan apprentices are abnormally small but chock it up to malnourishment. It turns out that Brokenstar is making three-to-five moon olds into apprentices. This turns out tragically when, during battle, a warrior attacks a three moon old named Badgerpaw and kills him.
- In Patience and Sarah, Sarah decides to move from Connecticut to York state by foot disguised as a man. She's twenty-one but her lack of facial hair and boyish look makes everyone guess she's fourteen at oldest. This leads to trouble because she's routinely mistaken for a runaway apprentice.
- Doctor Who: It's a Running Gag that the 12th Doctor (played by Peter Capaldi) constantly insists that Clara (played by Jenna Coleman) looks older than him, despite the fact that Capaldi is twice the age of Coleman, and as a Time Lord the Doctor is well over a hundred times older than Clara. A fan theory is that since Clara jumped into the Doctor's timestream to save him from an enemy that was attacking him at every moment of his life, the Doctor instinctively sees her as the same age as himself.
- In an episode of ER an injured toddler comes in and shouts that Uncle Charlie hurt him. Someone asks if they should call Social Services only for someone else to point at another, unharmed but still grouchy, toddler, and say "That's Uncle Charlie".
- In an episode of Firefly, Mal invokes this trope to evade I Never Said It Was Poison, when asked whether he has seen fugitives, "a brother and sister", by pretending to assume that the pair in question were children, when he actually knows exactly who and where they are.
- It's never explicitly stated what Niles expected in Frasier, but he is clearly surprised the first time he meets Daphne, mentioning that Frasier told him she was English and um, well... He most likely assumed she was a stuffy older lady.
Niles: You're Daphne?
- House mistook a 37-year-old patient to be at least 10 years younger than she actually was, which turns out to be a clue for her illness. She has "pretty leprosy", a form of leprosy that ended up making her look better, instead of making her skin fester and rot off.
- Charlie in NUMB3RS was once invited to a bed and breakfast in Santa Monica after publishing his first paper. Unfortunately, he was fourteen at the time and he had to break it off to a very embarrassed female professor in Berkley.
- In an episode of United States of Tara, Kate's boyfriend is about to meet Kate's family, and Kate had previously mentioned that she had a younger brother. Kate's boyfriend buys a toy train, expecting that he'll be quite young, except Marshall (Kate's brother) is in his teens, causing an awkward interaction.
- Hedley's "Don't Talk To Strangers" is about a man who has sex with a woman. The morning after, he realizes that she's twice his age and old enough to be his mother. He literally went to school with her son.
- Our Miss Brooks:
- In "Mr. Boynton's Parents", Mr. Boynton's Parents mistake a housecleaning Miss Brooks for old Mrs. Davis.
- In "Former Student Visits", Miss Brooks is desperate to appear young. She tries to hide the fact that she's already been teaching high school for several years. Unfortunately for her, the "former student" visiting had been in her very first class. He's since graduated from medical school and become a doctor!
- The Imaginary Invalid: Diafoirus the younger is presented to Argan's daughter Angelique (which he's intended to marry) and gives her the speech intended for his future mother-in-law. Although it's meant to show Diafoirus as a dumbass, Argan's second wife Beline is a Gold Digger, so it's quite possible she's around the same age as her stepdaughter.
- In Pokémon Sun and Moon, when Hau first meets Lusamine, who runs the Aether Foundation, Hau assumes she is a Teen Genius and admires that she can achieve so much in such a short time. Lusamine then points out that she's actually well over 40 years old, and she only looks so young because she knows the right things to wear.
- One of the races in Final Fantasy XIV is the Padjal, Horned Humanoids gifted with eternal youth. During the Dark Knight story, Sidurgu mistakes E-Sumi-Yan for a boy, but the Conjurer corrects the "young man" that he is several times his age. Indeed, Brother E-Sumi-Yan, according to Encyclopedia Eorzea is over two hundred years old.
- Granblue Fantasy: In her 5★ Uncap, there are some Human children who naturally mistook Charlotta as a fellow child. The scene may also double as a Rule of Funny moment due to the casual, yet playful manner of the children while talking to her. Perhaps they are not aware that they are talking to a Harvin (a dwarf race).
Charlotta: Wha?! I-I may look this way, but I assure you I'm an adult!
Child : C'mon, lady! You ain't bigger than my little sis! And she just started talking last week!
- In Tower of God this happens commonly since the Shinsu atmosphere slows down and sometimes even stops aging. This leads to cases of Shibisu being in his late twenties and Yeon and Hatz in their late teens whereas Endorsi is about 300 years old and her "niece" Anaak is even older.
- A Running Gag in Hanna Is Not a Boy's Name, as one of the few things that seems to really aggravate Hanna is when people mistake him for a teenager. ("I'm 24.") He certain looks young, but it's never made clear if that has a supernatural basis or if he's just naturally scrawny.
- Beloved starts with the protagonist finding out she slept with a sixteen year old girl she met at a bar.
- In True Villains, Sebastian assumes that the Lady of Black Magic Elia is in her mid-twenties like him, and is flabbergasted to hear that she's actually in her eighties — a perk of being a Necromancer who can steal a young woman's skin whenever she wants to restore her youth. It doesn't diminish his interest in her.
- In El Goonish Shive, Amanda exploits having drunk a soda with a Fountain of Youth formula by going up to an age guessing booth.
- In Alvin and the Chipmunks, Alvin hears about having an oldest fan, and thinks she's just in her twenties, while she is actually a senior citizen.
- Comes up in an episode of Family Guy, where Quagmire (who else?) meets a girl at Peter's party and sleeps with her, thinking she is 18, only to find out that she is 16. (Granted that is still the age of consent in Rhode Island, but who cares?)
- In the Steven Universe episode appropriately titled "Steven's Birthday", 12-year-old Connie (and the audience) is shocked to learn that Steven is turning 14, having assumed that he's at least a year younger than her due to his short stature and childish behavior; Steven's Half-Human Hybrid genes has affected his natural growth, and he has looked like this since he was eight.