Totally Eighteen

This character, or group of characters, is most definitely not underage. Nope, they are at least 18. That's what the narrative keeps pointing out, or what some character keeps claiming.

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.

In some cases, but far from always, this trope comes across as a Suspiciously Specific Denial of one kind or another. The trope is sometimes connected to tropes such as Jail Bait, Legal Jailbait and The 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.

Compare Older Than They Look and Really Seven Hundred Years Old as well as Younger Than They Look and Really Seventeen Years Old. For sexual situations Suspiciously Vague Age is another method writers might try to get around the no one under eighteen having sex thing.

Comic Book

 * Passionately averted in Lucifer with the protagonist Elaine. At the beginning of the story, she is twelve years old, and as she grows up her age is never mentioned again. She gradually and seamlessly transitions from childhood to becoming a Time Abyss.
 * 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 for forgiveness for her dishonesty, he breaks all contact with her and spends the rest of the episode angsting over having been with someone so young. The story takes place in Sweden, where the age of consent is fifteen—so in that context his angst is purely emotional, without any Jail Bait issues.

Film
"George: Eighteen! Why, it was only last year you were seventeen."
 * 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 whom he thought was eighteen.
 * It's a Wonderful Life, of all places. When Mary tells George she is 18, the implications weigh heavily on him.

"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-- Amy: No."
 * It is explicitly mentioned in Sucker Punch in regards to Baby Doll. She's 20 according to the script.
 * The "barely legal" genre of porn, where the stars are between the ages of 18 and 21 and look even younger.
 * Tangled could arguably be seen as a non-sexual variation—Rapunzel looks maybe fourteen, but her eighteenth birthday is a plot point, presumably to avoid the issue of her finding her true love as a teenager.
 * Aladdin and The Little Mermaid also take pains to mention the princess' ages, though in those movies they're eighteen and sixteen respectively.
 * Subverted in The Social Network:


 * Played straight later in the film, 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.

Literature
In the last few books, a young prince join 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 punishes him for covertly taking a slave before he's old enough... by sending him off to France... where sixteen-year-old nobles are allowed to own slaves... And yes, he gets to bring the former journalist with him.
 * 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 slave-owners eventually does becomes a plot point, however.
 * A prelude to this is given in the second book: some teenage nobles are frustrated and annoyed over not being allowed to own slaves yet, and a few sixteen-year-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.

Live-Action TV

 * Discussed in Californication when Hank Moody discovers the attractive socialite that seduced him at a book store (while reading his book no less) is 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.
 * Of course, in real life a girl who lied about her age and seduced him would not have much legal ground to stand on. A point which is raised when
 * 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 she happen to have a medical condition that make 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 was 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.

Video Games

 * 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 - specifically from a female player who felt like it was sexual harassment, since 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 a 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 her). When you kill one, they sometimes moan 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".

Visual Novels

 * Fate/stay night has a splash page at the beginning noting that all characters in sex scenes are of legal age. Due to half those characters being in high school, there is some debate over whether this is Blatant Lies or just taking advantage of Japan's slightly more complicated age of consent laws.

Web Original

 * 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.

Real Life

 * 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 they therefore have to do some work to actually remake their images into those of adults.