Mistaken Age

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. In many cultures aunts are thought of as old (and can even be offensive in Japan), so when the aunt turns out to be only a few years older than the niece/nephew, it can be quite a surprise.

This comes up often when a character is a Hot Mom or Hot Dad.

Compare Tomboyish Name, Expecting Someone Taller, Actually, That's My Assistant, Older Than They Look, Younger Than They Look, Age-Inappropriate Dress.

Examples of Mistaken Age include:

Advertising

The Grape Nuts cereal television ad: "Mrs. Burke, I thought you were Dale!"
- Later followed by the Palmolive's "Which hands belong to the daughter?"

Anime and Manga

In Tower of God this happens commonly since the Shinsoo atmosphere slows down and sometimes even stops aging. This leads to cases of Ship Leesoo being in his late twenties and Yeon and Hatsu in their late teens whereas Androssi is about 300 years old and her "niece" Anak is even older.
In Denno 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 Maria of Hayate the Combat Butler‍'‍s Berserk Button. 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.
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.'
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.

Comic Books

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.

Film

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 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.
In the Babysitters 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.

Literature

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.

Live-Action TV

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 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.
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.

Professional Wrestling

Eddie and Chavo (Jr.) Guererro were close enough in age to be brothers (2 years apart), but they were uncle and nephew.