Really Seventeen Years Old

A character lies about their age or hides their real age in order to get around a social or legal boundary. The reasons the character does this vary. Sometimes they feel a sense of duty and are stopped by that age boundary. Sometimes they feel too limited by their age and wish to hide it. Maybe they feel that they are above the law. Either way, the interest of the situation comes either from the character trying to keep the secret until they almost inevitably get caught, or sometimes in Rule of Funny situations, it comes from The Reveal and consequences of that reveal.

When it's about sex, the age boundary is virtually always 18 due to SoCalization.

Not to be confused with Younger Than They Look, where a character is much younger than they appear to be, but not because they are lying about it.

Truth in Television for many different contexts.

Contrast Really Seven Hundred Years Old. When the creators or the narrative are the ones lying to the viewers about the real age of the characters in order to show them having sex, it's Totally Eighteen.

Examples of Really Seventeen Years Old include:

Comic Books

Happens with a porn actress in the Ms. Tree storyline "Runaway II" (which was probably inspired by the Traci Lords scandal that was unfolding at the time. See Real Life examples below).

Film

In Animal House, one of the characters meets a girl and they have a fun romp which ends with them having sex in the middle of a football field. After all this is done, she reveals that she's thirteen. The guy is understandably horrified.
- This was a Censor Decoy that the censors somehow missed. The writers wanted them to "catch" this, then "allow" them to change it to sixteen as a compromise.
Whip It: The protagonist says she's 22 in order to be allowed to join the Roller Derby League. She's 17.
In Old School, the main character sleeps with a random girl at a party, and later learns that she is his boss' seventeen year old daughter.
In Don't Tell Mom the Babysitter's Dead, the oldest of the kids lies on a resume in order to get a better job than flipping burgers so they have money for the summer. She winds up as an executive assistant instead of the secretary position she was aiming for, and has to continually lie about her age (and experience) in order to keep getting away with it. She eventually confesses to being in high school at the climax of the movie, but she's done such a great job turning the company around that the investors (and her boss) don't care.
One of the major subplots in Superbad involves the characters trying to pass as 21 (when they're still in high school) in order to buy alcohol. Hilarity Ensues.
In Hot Fuzz, Nicholas throws a bunch of underage drinkers out of the bar, who lied about their real age to get in.
Liar Liar: Fletcher discovers that Mrs. Cole lied about her age when she got married, rendering the prenuptial and the original marriage contract void, but leaving her still entitled to half her (ex-)husband's wealth, as they had lived together long enough to be considered common-law married anyways.
In The Matrix Revolutions, the Kid is well, a kid, but tries to pass himself off as an eighteen-year-old in order to fight the machines in the docks. Captain Mifune tells him he should have said sixteen, as that might be believable. When the Kid cops to it, Mifune points out he's too young to join the corps... until the Kid points out that the machines would kill him no matter his age, which convinces Mifune to let him help.

Literature

Miles Vorkosigan, in his "Admiral Naismith" persona, is reputed to look seventeen because of advanced rejuvenation techniques. He is, in fact, seventeen.
In the Old Kingdom trilogy, when Sam starts flirting with Lirael, she discourages him by telling him that she's thirty-five years old (he's seventeen). When he expresses incredulity at this because she looks too young, she explains that she looks like she's only nineteen because she uses really effective youth-enhancing creams. He believes her. In fact, she looks nineteen because she is nineteen.
In the Time Scout series, Margo Smith shows up on Time Terminal 86 claiming to be eighteen and determined to be the first female time scout. About halfway through the book, she reveals that she's just celebrated her seventeenth birthday. After a passionate rendezvous with a man twice her age.
In Wild Magic (the first book of the Immortals quartet), thirteen-year-old Daine claims to be fifteen. The person she's lying to sees through it, though.

Live-Action TV

In Arrested Development, Maeby gets a job as a producer where she constantly has to hide the fact that she's still in high school. She deflects any comments about her age by acting flattered and saying "marry me!". At one point, someone from the family asks about her job, she reflexively tries the same deflection technique but only gets a confused response.
On Malcolm in the Middle, Malcolm beats up a bully, then gets in trouble when the bully turns out to be much younger than he looked. Hal then gets a visit from what appears to be the boy's father, and after he starts behaving abusively, Hal beats him up too. Turns out the "father" was actually a minor.
Ron Howard played a kid who lied to get into the army on M*A*S*H; Hawkeye caught him and sent him home.

Kid: I'll hate you for the rest of my life!
Hawkeye: Let's hope it's a long and happy hate.

In the first season of American Idol there was a guy who was a few years older than the maximum allowed; when they found out he was booted from the competition.
- In the first season of Idol in Norway, a 15-year-old girl lied about being 16. She made it through the semifinals and was disqualified once they found out.
A wannabe wrestler did this on Tough Enough (the WWE Reality TV show), when they found out he was kicked off the show but got hired by WWE anyway, becoming The Boogyman.
- An inversion, he was dropped for actually being older than the cut-off.
On Cold Case, one subplot involved a witness who was an army recruit who lied about his age and was almost discharged (for other reasons).
Occasionally pops up in the Law & Order franchise, especially Law & Order: Special Victims Unit.
When Laura married Petrie in the The Dick Van Dyke Show, she lied about her age. She told her fiance that she was nineteen when she was really seventeen and hence too young to get married.
One of the major story arcs in Californication involves middle-aged Hank Moody being seduced by a girl who neglected to tell him she was only sixteen until the next day.
One of Casey's dates in Greek turned out to be sixteen...and he had a fake ID good enough to fool Lizzi, the resident Party Patrol.
In an episode of Friends, Monica dates a younger guy who appears to be in his early twenties and claims to be a senior in school. After they sleep together, he admits that he's that he's not a college senior...

New Media

Olivia did this to join an army as a non-combatant cook in Descendant of a Demon Lord.

Professional Wrestling

The Extreme Championship Wrestling's infamous "Mass Transit incident". In 1996, Erich Kulas, a 17-year-old, 350-pound kid from New England with no in-ring ability, lied about his age, his wrestling experience and his schooling background (claiming to have been trained by Killer Kowalski) in order to get into the ring. He was given the gimmick "Mass Transit", a Ralph Kramden-like wrestling bus driver, and thrown into a tag-team hardcore match (filling in for Axl Rotten) alongside D-Von Dudley against New Jack and Mustapha Seed. It ended about as well as you would expect—Kulas left the match lying in a pool of his own blood after a botched bladejob, New Jack was arrested and later sued (he was let off after it was revealed how Kulas had lied to the bookers), and ECW's first pay-per-view, Barely Legal, was almost canceled, and wound up getting pushed back by four months.

Web Comics

Least I Could Do has a storyline where the main character meets a girl in a bar who turns out to be a minor.
Something*Positive: Helen (a crossover character from Penny and Aggie) tries to get a job at Nerdrotica despite being underage. Nancy catches her out because of her crappy fake ID.

Western Animation

Parodied in The Simpsons: Grampa Simpson says that he fought in World War I -- "of course, I had to lie about my age to get in." In the ensuing Flash Back, he's five.
King of the Hill: Cotton Hill states he was about sixteen when he joined the Army, lying about his age.

Real Life

The Chinese gymnastics team was accused of doing this in the 2008 Olympics and in other international gymnastic tournaments.
2001 Little League World Series pitcher Danny Almonte's father forged his birth certificate, saying he was 12 (eligible to play) instead of 14 (ineligible).
A common tactic employed by young men eager to join the army, especially in World War I...
- Also reversed. Men older than the 40 year old cutoff age often claimed to be in their late 30s so that they could serve.
- Allegedly, some boys would put a scrap of paper with the correct number (16 or 17) written on it in their boot, so they could say they were "over 17" without it technically being a lie. This trick may have helped some of the boys who tried it. When you're lying, especially when it's on the spot, you tend to hesitate when you're speaking because it takes longer to think up a lie than it does to just think of the true information. When you lie, you're speaking faster than you can think. As well, there are also some body language cues that a well-trained person can use to infer that the person is lying. It being technically true that they were over a certain number may have helped these boys not give themselves away through these cues, especially if they looked the part as well.
Laurence Fishburne lied to the producers of Apocalypse Now to get a role in the movie. He said he was 18 when he was really 14. Coincidentally, his character was seventeen years old.
- And the film took so long to complete that by the end of the shooting he actually was 17.
Traci Lords became a porn star at the age of 16, using a fake ID. Authorities discovered this two years later, and almost all of her films were removed from distribution. To this day her early films are illegal to even look at (much less distribute), because they became retroactively classed as child porn.
- Her lie resulted in several hoodwinked men being sent to prison and shook the industry to its core. The result of the Lords scandal is that the porn industry is now extremely thorough in their background checks (they make damn sure all their actors are at least 18). It also resulted in changes to some processes in the Federal Government, which had issued her a valid passport supporting the fake ID...
Mila Kunis was 14 when she auditioned for the role of Jackie on That '70s Show. She told the casting directors she'd be 18 on her birthday, but didn't say which one. This made her the actor on the show closest to her character's age.
Edgar Allan Poe married his 13-year-old cousin, Virginia Clemm. To get around the age issue to get the marriage license, they had some papers forged that gave her age as 21.
The "Rubygate" scandal in Italian politics has this in spades. Browse The Other Wiki for details.
Gillian Anderson was twenty-four when she auditioned for The X-Files, but she knew Scully was a doctor so she said she was twenty-seven.
Inverted by Kikuko Inoue as her Running Gag. She's already a married woman, way above 17 years old, but she still constantly refer herself as "17 years old".
Similarly, Jack Benny pegged his age at 39 as a Running Gag that he kept up all the way to the end of his life.
Zahia Dehar, a French prostitute who slept with several high profile footballers (among others), have done so under the age of 18. Her clients deny that they were ever informed, which she has confirmed several times.
Audie Murphy, in a literal example. He was seventeen when he joined the US Army after his sister altered his birth certificate. His tombstone in Arlington National Cemetary states his false date of birth, however.
A repeat of the Traci Lords scandal occurred with the Full Motion Video adult quiz game The Guy Game, four months after the game's release. One of the models filed suit against Topheavy Studios, Gathering of Developers, Sony and Microsoft, explaining that she was not informed that the footage would be used for the game, and that she was seventeen years old, thus giving The Guy Game the dubious distinction of being the one of the few (if not the only) works of underage pornography to be licensed for a home console.