This can be used to add a sexual subtext to an otherwise non-sexual situation, since works with sexual content often use this trope as a disclaimer.
When done in-universe it's usually framed as a Suspiciously Specific Denial of one kind or another, though keep in mind that it's actually true just as often as not. The trope is sometimes connected to tropes such as Jail Bait and Jailbait Wait for the character, or tropes such as Moral Guardians and Getting Crap Past the Radar for the work itself.
Note that in many countries and states, the age of consent for having sex is fifteen or sixteen while the age when it becomes legal to be portrayed as having sex is eighteen. Regardless of laws, many individuals draw a very sharp moral line at eighteen for all things sexual. Also note that while often directly or indirectly sexualized, the trope does not need to have any sexual context or subtext - the driving plot point can also be issues such as the right to vote or to get your driver's license.
Then there's the "barely legal" genre of porn, where the stars are between the ages of 18 and 21 and look even younger.
- In one issue of Mats Jonsson, the protagonist is having a relationship with a girl who pretends to be 21. When it's revealed that she's studying at high school rather than the university, she changes it to nineteen. When she later confesses that she's actually sixteen and begs forgiveness for her dishonesty, he breaks all contact with her and spends the rest of the episode angsting over having been messing around with someone so young. The story takes place in Sweden, where the age of consent is fifteen - so his angst is purely emotional, without any Jail Bait issues.
- Cherry Comics regularly goes out of its way to point out that Cherry and her equally promiscuous school friends have "just turned 18."
- The Torture Porn comic house Fansadox includes multiple disclaimers in its web and comics claiming that all characters are "at least 18 years old". If said comic "stars" The Ingenue and shows her in high school, the narration and dialogue will tell you several times that she just turned eighteen. For some reason, this make it perfectly okay to rape, torture and often murder her.
- Played with in the season 1 finale of Children of Time. Beth Lestrade is seventeen as "Dynamics of a Point" begins, but due to wibbly-wobbly, timey-wimey stuff, she's truly eighteen in "Every Good Fairytale" (when she does marry Sherlock Holmes. In the original transcript, she described herself as a seventeen-year-old during the honeymoon, and Sherlock pointed out that she was a fully mature adult as far as anyone was concerned. This was later altered when Beth's writer realized some convenient but no less real implications of Beth being a Temporal Paradox.
- In Brain Donors, at one point the trio is awakened suddenly, and the first thing out of Roland T. Flakfizer's mouth is "She looked eighteen, officer, I swear!"
- This is part of the reason why R. P. MacMurphy in One Flew Over the Cuckoo's Nest is locked in a mental institution; he had sex with an underage girl he says he thought was eighteen.
- It's a Wonderful Life: When Mary tells George she is 18, the implications weigh heavily on him:
George: Eighteen! Why, it was only last year you were seventeen!
- It is explicitly mentioned in Sucker Punch in regards to Baby Doll. She's 20 according to the script.
- In Tangled, Rapunzel could easily be mistaken for being much younger than she is, but her eighteenth birthday is a plot point, presumably to avoid the issue of her finding her true love as a teenager.
- In a rare under-18 example, The Little Mermaid (1989) makes a point of establishing Ariel as 16. Since King Triton (eventually) consents to her marriage to Prince Eric, this would (just barely) be legal in most places in the modern age; in vaguely-old-timey Denmark (or wherever; it's not clear if the setting is the same as Hans Christian Andersen's original story) it's presumably perfectly normal.
- The Social Network: Multiple:
- Subverted with Amy and Sean:
Amy: (after discovering Sean, whom she just slept with, doesn't go to college) Seriously, you're not like 15 years old or anything, are you?
Sean: (chuckling) No, I'm 22. (Beat) *You're* not like 15 years old-
- Played straight when the cops bust a party Sean is throwing. One of the girls at the party pretends she's 19, but when pressed, admits she's really 17.
- Subverted with Amy and Sean:
- Inverted in Barbie in the Pink Shoes: While they make a point about Kristyn being only 17, it's because she's trying not to be Giselle. Albrecht and Hilarion don't realize that she considers herself too young for marriage, and instead consider her too old for marriage.
- Ever since the infamous "pedophilia how-to guide" incident on Amazon.com late in 2010, Amazon.com has been rather squeamish about selling any erotic literature featuring underage sex and/or incest. This led to a number of erotic authors' stories being yanked from distribution with no warning and little to no explanation. Since then, many erotic authors such as Selena Kitt have lately reworked a lot of their stories to specify that everyone in them is totally eighteen or over; also that they're totally Not Blood Related. Almost invariably, any stories about—for instance—"daddy's little girl" will have the girl saying something to her daddy about how she can decide things for herself since she's eighteen now, and will mention in passing or work into the plot some reason why her biological father is gone and this guy is actually her step-daddy. So it's OK for them to pork.
- In the Slave World kingdom of England, you have to be at least eighteen before you can become a slave or own a slave. The first part is merely mentioned (especially in connection with those few slaves who are exactly eighteen - most are significantly older). Non-noble characters younger than eighteen simply doesn't exist in the story. The rule against underage slaveowners eventually does becomes a plot point, however. A prelude to this is given in the second book. It is mentioned that some teenage nobles are frustrated and annoyed over not being allowed to own slaves yet, and a few sixteen years old nobles fondle the protagonist while she happens to be helpless. Around the same time, a young adult noblewoman is scolded for being immature and reckless, not taking proper care of her slaves. In the last few books, a young prince joins the cast as the Love Interest of an enslaved journalist. He is sixteen, she is in her thirties, but because of the social power dynamic, he's the one with all the power. The queen eventually punish him for covertly taking a slave before he's old enough... by sending him off to France... Where sixteen years old nobles are allowed to own slaves... And yes, he gets to bring the former journalist with him.
- Discussed in Californication when Hank Moody discovers the attractive socialite that seduced him at a book store (while reading his book no less) is actually 16... and the daughter of his ex-girlfriend's current partner. The mistake is a major plot point for the rest of the series as the girl in question keeps the threat of blackmail pointed at Hank.
- Law & Order: Special Victims Unit sometimes treats the fact that a certain character is over 18 as an annoying technicality that makes it harder to arrest people for having sex with them:
- In one episode, the sex is consensual acts between two adults who love each other and are both fully mature in every mental, emotional and legal way. It's just that one of them happens to have a medical condition that makes her look like she's ten. The detectives consider her chronological, mental and emotional maturity to be an annoying technicality, one that they try to find ways around so they'll be able to lock up her current lover and any lover she may get in the future. Instead of taking the actual person into account, they put all their focus on how grossed out they personally were over imagining anyone being turned on by her. So they basically wanted to follow this adult woman around for the rest of her life to harass her and anyone she might fall in love with, simply because of her looks.
- In another episode, a girl is raped at gunpoint. She looks very young, and throughout the episode she is is consistently portrayed as a teenager who is not yet fully adult - neither intellectually nor emotionally. This is not held against her; instead, it simply underscores how vulnerable she is. However, she happens to be 19, so the prosecution must prove that she didn't consent. And of course, the defense has Blatant Lies about the gun as one of their top priorities.
- Invoked in-universe in an episode where they find out their teenage witness for a case is actually an almost 30-year-old woman who's been successfully posing as a high schooler for over a decade. Throughout the episode, the detectives and ADA try to get her to admit the truth, especially when facing possible statutory rape charges, but she's still in denial of her actual age. She eventually does when confronted with her first foster mother.
- That '70s Show: Meta-example. When she first auditioned for the part of Jacky, Mila Kunis was fourteen. When the producers asked how old she was, she answered "I'll be eighteen on my birthday," and they failed to ask which birthday. Thankfully, the job didn't require her to do anything that would be illegal for a minor, so she dodged most of the problems that come up in this situation.
- On Friends, Monica shaves a few years off her age (dropping her into the early 20's) then finds that the man she just made love to was a high school senior, not a college senior as he told her. When he defends himself from her anger by saying she lied too, her response is "Well, my lie didn't make one of us a felon in 48 states!"
- In Veronica Mars, Keith Mars shows Sheriff Don Lamb a photo of Lamb making out with Madison Sinclair, a classmate of Veronica, in order to get information out of him. Lamb tells Keith "she's 18 - that's legal", but capitulates when Keith challenges him to wait and see if that explanation is good enough for his constituents when it comes to his re-election as sheriff, and shows another copy of the photo with "18 - that's legal" printed on it.
- In World of Warcraft, the spring festival Noble Garden includes an achievement where you are supposed to put bunny ears on one female character of each race, and it has to be a character that is at least level 18. Some complaints about this achievement made it into the newspapers - some female player who felt like it was sexual harassment - other achievements about putting hats or whatever on other characters was not a problem, but the "level 18" requirement made this one sexualized in her eyes. The actual reason it was added was to prevent players from trivializing the achievement by making level 1 alts, as some had done with an earlier event (that it was 18 specifically was probably just Rule of Funny).
- The game Bear Gunner is technically about hunting bears. However, the protagonist is an 8-year-old girl armed with a machine gun, and the bears she's gunning down are all the pedobear logotype and busy chasing little girls (including you). When you kill one, he sometimes moan some dying words, such as "Argh!", "I see a bright light", "why me?", "I'm just a guy in a suit" or "I swear she was eighteen".
- In Evil Zone, Alty Al Lazel, Erel Plowse, Midori Himeno and Setsuna Saizuki were all between 14 and 18 years old in the Japanese version, but the localization boosted their ages to 21. Linedwell Reinrix's age also went from 20 to 22 for some reason. The translated manual nevertheless lists the characters with their original ages.
- Yandere Simulator comes with a disclaimer that all characters are 18 or older, regardless of the creator's statements that the protagonist — a second-year Japanese high-school student — is 17. Since characters can be nude (albeit with Censor Steam) and taking Panty Shots is a game mechanic, the developer felt the need to stay on the safe side.
- Parodied in Danganronpa Another Episode: Ultra Despair Girls where at the end of elementary-school girl Kotoko Utsugi's mini-boss with Genocide Jack (in which she gets stripped down to her underwear) she will claim that it's upsetting, but it doesn't matter since she's eighteen. Not helped by Kotoko's horrific backstory.
- In the first few Dead or Alive games, several characters didn't have their ages listed in the American releases because they were underage but still delivered just as much (if not more) Fanservice as the adult women. DOA 4 had a Time Skip that put the entire cast at 18 or older, meaning suddenly it was okay to print their ages in the American versions. Plenty of fans suspected that this trope would be in effect in DOA 5 with Marie Rose thanks to her Elegant Gothic Lolita design and her having the least developed figure out of the franchise's women, but she actually is 18 years old.
- A slight variation occurs in Fire Emblem Fates, as no specific ages are given. However, characters still insist that Elise and Midori are really adults in spite of both of them looking quite young (Leo's infamous "technically an adult" line for Elise, and Midori herself in a conversation with her mother). Therefore, it's totally okay for them to get married and have children.
- In Max Payne 3 following a shootout in a strip bar/brothel, an American tourist hiding from the mayhem tells Max that he likes this place because it has the cheapest hot women he's ever seen, including some who insisted on being eighteen years old. In a later level, as Max shoots his way out of jail, he sees the same tourists who keeps insisting that they told him they were eighteen.
- All Western-released materials of Fate/stay night come with a disclaimer that all of the characters are 18 or older, despite Shirou, Rin, and Sakura being in their second year in high school making them between 16 and 17 in the Japanese version with Saber being physically a year younger.
- Parodied in Namco High, whose legal page has very faint text at the bottom, reading "18 18 all characters 18 everyone 18" despite the fact that Namco High is entirely free of adult situations.
- Despite being ostensibly set in a high school, the main cast of Monster Prom is somewhere in their 20s or older, with the youngest character being 19 at the least. Normally, this might be attributed to monsters aging slower than regular humans, but then Liam specifically mentions age shipping discourse brought up in Star vs. the Forces of Evil fandom,note then notes they don't have to go through that discourse since they're above legal age. And given the high amounts of horniness and sex that happens in the game, this in-universe disclaimer might have been in the developers' best interests.
- Our Little Adventure: The main reason why The Rival's wizard is a wanted criminal; apparently, 18 years old in Halfling years is NOWHERE near adulthood or even puberty.
- In El Goonish Shive, when Raven tells Grace he doesn't want to drag her, a minor, into a dangerous situation, Grace corrects him:
Grace: I'm eighteen! I'm a major!Raven: As though that makes you an adult.Grace: Legally it does. You can do all sorts of stuff with me!
- In the Honest Trailer for the Harry Potter movies, the narrator immediately backtracks after referring to Hermione as Harry's "superior-in-every-way hot lady friend" by adding "And when I say hot, I mean only in the last three movies after [the actress playing her] turned eighteen! Heh heh... pervs!"
- On Literotica it's considered wise policy to specify that any characters engaging in sex are over 18.
- This is mocked by Zack Parson of Something Awful when he did his Hentai game reviews, some of those games claiming someone who in his eyes is obviously a minor is in fact 18.
- Similarly, due to a crackdown on Tumblr blogs containing cartoon pornography depicting underage fictional character(s), many artists have begun writing a disclaimer that "all characters depicted are at least 18 years old or otherwise above the age of consent", regardless of whether or not the characters actually do look 18+.
- In one issue of Teen Girl Squad, Cheerleader, hoping to impress some college-age guys, insists she and So-and-So are in college. So-and-So, however, undermines this by simultaneously admitting to being in eighth grade. Cheerleader tries to backpedal by claiming that So-and-So is her younger sister by five years, but the boys don't buy it (and then she's hit by a SAILING MISHAP!)
- A certain pornographic website, frequently advertised on Comedy Central, announces that its actors are 18 right in the URL.
- In Clone High, the underage clone of Abraham Lincoln recruits the underage clone of Genghis Khan to buy beer for a party, telling him all he has to do is tell the clerk he's 21.
First Clerk: You over 21?Genghis Khan: (stares blankly)(outside another store, Abe coaches him)Abe: Okay. He used a very tricky line of questioning. Let's practice again. Are you over twenty-one?Genghis Khan: Yes.(inside the store)Second Clerk: Are you over 21?Genghis Khan: No.(inside another store, Genghis Khan has a large sign affixed to him that reads "I AM 21")Third Clerk: So, I guess you're 21?Genghis Khan: I am 21.(the clerk gives him the keg of beer and he walks off, then immediately runs back in.)Genghis Khan: I AM NOT 21!
- Those who have very recently turned eighteen (or their country, state, or province's age of majority) will often be very eager to point it out as well as take advantage of the new privileges and rights they have. It's justified because since their becoming of the age of majority, people around them will most likely see them as "just a kid" and therefore they have to do some work to actually remake their images into those of adults. Meanwhile, any establishment that has an age requirement for its patrons tends to make it a policy to card anyone who doesn't look 30, just to be sure.