Petunia Dursley: No.
Dumbledore: I'm sorry?
Dursley: No, he doesn't. He's a month younger than Dudley, and Dudders doesn't turn eighteen until the year after next.
Dumbledore: Ah, but in the Wizarding world, we come of age at seventeen.
Age-based thresholds are important both in fictional settings and in the real world when people become something upon hitting their nth birthday. For example, becoming an adult is an important step in growing up, and in Real Life and often also in fiction it is defined by a specific age. At the Fictional Age of Majority the character is old enough to be either considered an adult, join the army, bear the crown, have their superpowers manifest, drive a car or some other plot-relevant thing. A story where this trope is in play is often set at the character's corresponding birthday. Often hitting this age leads to a Rite of Passage.
- In Attack on Titan, the minimum age for enlistment is 12 years old. Training is a 3-year period, so it isn't clear whether one is considered an adult at enlistment or at graduation. (Assuming they survive to graduate, that is.)
- Black Clover: Every year, fifteen-year-olds in the Clover Kingdom go to towers to receive their grimoires, which greatly enhance their magical ability. This ceremony kicks off Asta and Yuno's journeys in becoming Magic Knights.
- In the Pokémon, or at least in the Pocket Monsters: The Animation universe, 10-year-olds can go off on Pokémon adventures, be imprisoned for crimes, and even get married. The exact level of independence they have in other universes is debatable and varies wildly.
- Depending on the Writer, the 15-16 year old heroes of Legion Of Superheroes are sometimes considered young adults by the United Planets. A plot point in the Reboot Legion was that Winath has a higher age of majority than the norm, meaning that Live Wire and Spark were still legally children when they joined.
- A.A. Pessimal's Discworld has Ankh-Morpork follow British custom and law, as a Fantasy Counterpart Culture of Great Britain (specifically England). The age of majority at which a young person is legally counted fully adult is when they turn eighteen. By default, other named countries which have a "post-colonial" relationship with Ankh-Morpork such as Fourecks, the Foggy Islands and Rimwards Howondaland follow this lead. However, the various provinces and states of the Semi-Untied States of Aceria vary between eighteen and twenty-one. Ankh-Morpork has no minimum age of consent for marriage or for smoking tobacco, but the Guild of Gamblers insists on a minimum age of sixteen and the Guild of Seamstresses will not employ people under eighteen. The Seamstresses are strict about this.
- Anthropology: Ponies naturally mature at a faster pace than humans, and reach their age of majority much earlier — Lyra left home at age twelve, which is stated to be a normal time for this to happen, and at sixteen years old she's well into maturity. This causes some severe misunderstandings once she is turned into a human and moves to Earth, as she doesn't realize what human growth patterns are like and ends up leading her new friends into thinking that she's a runaway whose parents kicked her out as a child.
- The Desert Storm: For Mandalorians, 14 is generally agreed upon as the age of adulthood where one is deemed old enough to fight in battles. Some clans go even further and have the age of adulthood set at 10.
- Pokémon Reset Bloodlines ups the age of trainers, and thus your own adulthood, at age 15. A few things can vary from region to region though: the drinking age for alcohol is at the same age in Johto, but in Kanto it is a year later at age 16.
- In Frozen, it's suggested that 21 is the age of majority in Arendelle; according to Jennifer Lee, Elsa is 21 years old during the main events of the story and although her parents died when she was 18, she isn't formally crowned queen until she "comes of age" three years later.
- Ascendance of a Bookworm:
- Adulthood ceremonies happen around fifteen years after one's birth. The new rights acquired after the ceremony include no longer needing parental approval do to things and getting married. It's also the age at which people are expected to settle in their permanent workplace (vocational training starts at age seven) and graduate from the equivalent of a temporary employee to the equivalent of a full-time one if it hasn't happened earlier. Some trades require the person to have proven that they can bring money into their new workplace by completing a special order for a customer by the time they reach adulthood.
- Children have their baptism around their seventh birthday. During that ceremony, children enter their local Temple for the first time and are officially made citizens of the city in which they were born. All children officially start their vocational training after their baptism. Getting orphaned or running away from home after one's baptism is also just slightly less painful than before it, as employers are expected to have housing space available for employees and apprentices with no other place to live. Working pre-baptism children are akin to child labor in modern times in that they're technically illegal by Temple rules, tolerated if they're children helping with their parents' business, and paid less than a seven-year-old just getting started. It's also mentioned that children aren't considered human beings by the Temple before their baptism. All of the above is problematic for Myne because she's still as small as a five-year-old long after her seventh birthday has passed, causing people who first meet her or only know her by sight to not realize that she has already been baptized.
- The Pocket Monsters: The Animation Light Novels from Takeshi Shudō takes it further regarding 10-year olds in the Pokémon world, both games and anime. Ten-year-olds who completed elementary school are considered legal adults with all the responsibilities that entails.
- Dark Skies: In Mudamora at least, the age of majority is 18, as princess's Malahi's 18th birthday is a very big deal. In Celendor it is implied to be much lower, as 18-year-old Lydia is said to be past the time to get married.
- In The Dark Tower, there's no fixed age of majority for a gunslinger; rather, you're considered of age if you have managed to win a fight against your fighting instructor with any weapon other than a gun. If you win, you become a gunslinger, and if you lose, you are "sent west" into exile. There's no fixed age for when the trials take place, but generally if you don't take the test by the time you turn 25 then it's assumed you never will take the test and will instead become a freeholder. Roland is the youngest to ever have passed the test, at age 14 (his father did so at age 16), while most gunslingers take their tests at around age 18-20.
- Deryni: In the first book, Kelson reaches his majority at age fourteen. It's a plot point that he was born in the afternoon, so he comes of age (and thus becomes king in his own right) the day before his formal coronation.
- Earth's Children: Neither the Clan nor the Others have a set age of majority. Instead, anyone who has reached puberty, which can be as young as seven or eight years old for Clan people, is considered old enough to be initiated into adulthood. Among the Clan, boys become men once they have made their first kill on a major hunt and girls become women once they have spent time away from the protection of their clan. For the Others, adult status is conferred by having a slightly older person introduce the youngster to sex, though the pair are strictly forbidden to form any emotional attachments.
- In the Emberverse, one must be 26 to assume the throne of either the Portland Protective Association or the High Kingdom of Montival. The age limit for the former was established as a provision of the treaty made at the Field of the Cloth of Gold, and it was adopted by the latter when the High Kingdom was established some years later.
- The Giver begins with Jonas going through his Ceremony of Twelve, in which twelve-year-old members of his community are given their "Assignments" (i.e. jobs) and become legal adults. The Film of the Book gives the characters an Age Lift and changes the age of majority to 18.
- The Han Solo Trilogy: Alderaan and Corellia both have seventeen as the age of majority. Winter and Bria note this in conversation, while the former feels it's too young as she had no sense then. At that age, Bria ran away from home and joined a religion which turned out to be a front for slavery, nearly leading her into being a sex slave for the rest of her life.
- Harry Potter: In the magical community, 17 is the age of majority. Among other things, this means finally being free of the Trace, a spell that tracks all use of magic near those under 17, to enforce the laws against underage use of magic outside of school.
- Hive Mind: Eighteen is the age of majority in Hive England, and presumably the other 106 Hives around the world, but with a twist; eighteen-year-olds aren't considered adults until they've been through Lottery. This is a massive event where all the eighteen-year-olds are tested before being imprinted with the information for their ideal career, thus saving years of training time and making sure people are matched to the right job.
- In The Inheritance Cycle, the age of majority in Alagaësia is 16. The main character, Eragon, has his 16th birthday in the first book, but lets it pass without comment due to his grim circumstances at the time.
- The Lord of the Rings: Hobbits legally come of age at 33. This is a function of hobbits' long average lifespans; they typically live to around 100. There are also cultural differences behind it; a hobbit in their twenties (or "tweens") isn't that much different developmentally from a human of the same age, but to a people that values a stable, simple life to such an overwhelming degree the passions and exuberance typical of that cohort seem much more childish.
- Riddley Walker: The age of majority in the eponymous character's society is 12 and the story begins on his 12th birthday.
- In Septimus Heap, would-be queens receive a red book on their fourteenth birthday which enumerates their political powers and responsibilities.
- Vorkosigan Saga: The age of Consent on Beta Colony is either 12 or 14. Cordelia mentions it twice in passing, giving different ages. Possibly both ages are significant.
- In the Warrior Cats series, kits begin the transition to adulthood when they are six months old, the feline equivalent of pre-teens; this is when they become apprentices, training under an older cat, and their names are changed from ___kit to ___paw. They are considered fully adult when they earn their warrior names or become full-fledged medicine cats.
- In Star Trek, a Klingon male is considered an adult when "he can first hold a blade". Later, it is stated that Klingons must go through the Rite of Ascension before they are 13 years old if they wish to become warriors. Klingons also physically mature faster than humans; Worf became a senior officer on the Federation flagship in his early 20's, for instance.
- In Babylon 5, Centauri are said to come of age at sixteen.
- Dungeons & Dragons: Some editions specify when the different races are considered adults for the purpose of entering a Character Class, which is usually proportional to their lifespan. For example, 3.5 Edition humans reach majority at 15, whereas elves, who can expect to live into their 500s, do at 110.
- Eclipse Phase: Rimward mentions that the age of majority in the Titanian Commonwealth is 25 for cognitively baseline transhumans. The implication being that the age might be lower for cognitively enhanced transhumans, uplifts, or AI.
- Warhammer Fantasy:
- Humans in the Empire, which is (very loosely) inspired by the 16th-Century Holy Roman Empire, are generally held to become adults either around age 16 or when they begin puberty. Some regions also make this the time to become engaged, to many newly pubescent "adults'" embarrassment.
- Dwarfs generally live over 200 years and can reach 400 or more. They become full-grown by 20 but aren't considered adults until around age 30, where they hold a Rite of Passage to make an offering of the craft they've been apprenticed to.
- In most entries of Pokémon, age eleven is the moment trainers are able to own their own Pokemon and journey around the world without supervision.
- The Legendof Zelda:
- In The Legend of Zelda: Ocarina of Time, Adult Link is 16 years old.
- In The Legend of Zelda: Majora's Mask, there's no specific age, but if you're old enough to be able to carry a weapon, you're considered an adult and are free to leave Clock Town, which is the case with Link, who is around 9-13 years old.
- The Legend of Zelda: The Wind Waker implies 12 years old to be at least a "special" age on Outset Island.
- In The Legend of Zelda: Breath of the Wild, the age of majority is implied to be 17, since the holy Spring of Wisdom is barred to all under the age of 17 years. Zelda herself turns 17 and plans to visit the Spring to help unlock her sealing powers, but that very day is when the apocalyptic Great Calamity occurs, underscoring the excessive pressure that has been placed on the princess to fulfil her destiny.
- In Splatoon, the age of 14 is when Inklings are considered fully developed and allowed to participate in Turf War battles. It is also around this time that they can fully control their ability to shift between their human and squid forms at will.
- Yes, Your Grace: At some point of the game, it turns that the reason Lorsulia's attitude has recently become nastier is that she expects to be married off anytime now that she has turned thirteen. Asalia, who's only about a year younger than her, treats this as a less immediate problem for herself, but can still be married off if the player chooses to do so.
- Arthur, King of Time and Space
- In the space arc, Guenevere and Morgan object to Merlin calling them "girls", saying "We're over fourteen and legally adults!"
- In the contemporary arc, an out-of-continuity discussion on whether Arthur and Gawaine should have been shown drinking alcohol considers the possibility that Camelot is in a US state where the drinking age is eighteen, before concluding "the contemporary time zone was supposed to be devoid of fantastic elements".
- Lovely People: People get their own social credit accounts when they turn fifteen years old. Before that, they depend on those of their parents.
- Avatar: The Last Airbender:
- Males in the Southern Water Tribe are taken by their fathers ice dodging at the age of 14 as a rite of passage into manhood, where they have to sail their boat around glaciers.
- The avatar is only informed of their chosen one status at 16 so they may begin training. Aang is considered to have been too young to begin his at 12.