Somehow, the social order has gone all topsy-turvy. Instead of the old having power over the young, the young are free to do as they will, and may even have power over the old. This is pretty much inevitably portrayed as a bad thing, regardless of the state of the world today with the old having power (which of course is exactly the way it was when those who are now old were young). The young in question might attempt to build some sort of new society, or they might degenerate intocomplete barbarism, or there might be a conflict between the former and the latter. The most famous example is Lord of the Flies, but it's not too uncommon, since it's a useful device to explain why teenagers are going on whirlwind adventures. Teenage Wasteland settings tend to be rather dark, since they often involve children and teenagers fighting for their lives, and quite possibly doing horrible things to each other. Children Are Innocent has no place in a Teenage Wasteland — not all Teens Are Monsters, but in a Teenage Wasteland, you're likely to find at least some that are. In extreme cases, The Apunkalypse may be nigh if the youths run amok or the youths are the only ones left After the End.
There are a few ways this might come to be:
A group of teenagers are lost, stranded, or otherwise isolated from society (e.g. Lord of the Flies).
Adults simply do not exist any more. They are gone, either because the children have instigated a teenage revolution and enslaved/killed all the adults (e.g. Children of the Corn), or because of something that was Only Fatal To Adults (Shades Children).
A society where the old nominally still hold power, but groups of youths have become too powerful to be truly controlled. (e.g. A Clockwork Orange).
A society where the young and the old coexist, but the young call the shots (e.g. Logans Run).
The notoriously violent British comic of The Seventies, Action featured a story where few adults remained, mostly unlovable fascists, entitled Kids Rule O.K.
The 1972 cult film Gas-s-s-s, an industrial accident releases a poison gas that kills everyone on earth over the age of 25. In the rest of the film, the main characters have to learn to survive on their own.
The 1968 film Wild In The Streets, which produced the Breakaway Pop Hit "The Shape of Things To Come", revolves around a 16-year-old pop singer being elected President via The Power of Rock, and bringing about a society where the young rule by forcing anyone over 35 into re-education camps and dosing them on LSD.
In Mad Max Beyond Thunderdome, Max discovers a fertile valley where the children of plane crash survivors have been left alone while the adults went to find help. It being After the End, there was no help to be had, and the kids wound up having to raise themselves.
In Children of the Corn, a group of children slaughter all the adults in their town and proceed to establish a primitive tribal society which functions at the behest of a strange god called "He who walks behind the rows".
The Warriors appears to be one of these, where the street gangs of New York (mostly teenagers and perhaps 20-somethings) openly rampage through the streets, even during the police's attempts at a crackdown. Cyrus, the "President" of the Gramercy Riffs, the largest and most powerful of these gangs, was attempting to unite the various gang factions.
Village of the Giants: Some teenagers consume an unknown substance and grow to 30 feet tall, then take over a town.
Diario de la Guerra del Cerdo (it was translated as "Diary of the War of the Pig"), a 1969 novel by Argentinean writer and Cervantes Prize winner Adolfo Bioy Casares. A dystopic world where old people are deemed as "useless pigs", any kind of healthcare or benefits has been removed by the government and youngster's mobs are given tacit right to kill them in the streets, forcing them to hide and live a miserable existence in the underworld. Thus, this trope; though the approach to this subject is both grim and comedic at the same time.
Shades Children: Only children are left, except for Shade himself, and he's... well, unusual. All other adults have disappeared. Shade treats the children in his care like soldiers, but they all trust him because he's older than them. In a world where most people don't survive beyond the age of 14, it is shown that people generally sort themselves into a hierarchy based on age, with the main character practically falling down to worship Shade when he first sees him.
In Larry Niven's "A World Out Of Time", most of the Earth was ruled by immortal boy-children who kept a supply of grown-ups around as breeding stock (The immortal girl-children were wiped out by a gender war and environmental changes making their territory uninhabitable). All new boy-children are taken from the adults and join troupes of the immortal boys; the ones that demonstrate "superior qualities" are sent back to the adults to become new breeding adults, while the rest become immortal and stay boys forever. Girls remain with the adults and grow into new breeding adults.
Both the boys and girls are depicted as cruel despots, but not because of their "youth"; most were far older than the adults and had the mentalities to match. They're cruel because they're powerful, ancient immortals, and cruelty is how they alleviate their boredom.
In the second novel in the His Dark Materials series, Lyra visits a world where a lot of the adults have been killed off by Specters, monsters that can only harm and be seen by those who have reached puberty. Much of the world is covered in abandoned cities left to gangs of spectre-orphans (until they grow up and get spectre-eaten themselves) while caravans with adults try to keep ahead of the spectres.
Evil by Swede Jan Guilliou is set in a 1950s boy's boarding school in which the boys are given to govern themselves in their lives outside the classroom - at one point the principal steps in to stop the beating of the main character during lunch, but only because splatters of blood land in his food.
A. E. van Vogt's Children Of Tomorrow: so many men have gone to war that there aren't enough left on Earth to enforce the law, and the children are organized into "outfits" with police powers.
Robert A. Heinlein's Tunnel In The Sky features a "stranded" situation, but it works out because they were being taught survival techniques anyway.
Charlie Higson's The Enemy, in which fourteen years ago, all people were infected with a disease which took this long to develop. Only people born since then remained. The adults either died or went mad and became cannibalistic zombies.
The nation-state of Canyonar in Waking Echoes by Donaya Haymond becomes a Teenage Wasteland, as a result of The Virus that is Only Fatal To Adults and Prepubescent Children. A few of the teens strive for order and peace, but because the place was already a Crapsack World before the Famine Fever hit, most of the kids either become cannibalistic looters or join a Fascist-style army with the aim of conquering the currently unaffected neighboring countries.
This apparently happened in the backstory of Timothy Zahn's A Coming of Age, which takes place on a colony planet where some unknown environmental factor gives preadolescent children powerful telekinetic abilities. This led to an extremely destructive period known as the Lost Generation; by the time of the novel adults have reasserted control, but only by isolating the kids and strictly limiting their access to information.
The Fire-us Trilogy has a (subverted) example, by due to a virus that killed off all the adults. Nearly all of the children also died without the care of their parents, but the survivors are all children or young teenagers.
There was an early episode of Sliders where Howard Stern had been elected president and lowered the voting age to nine. The young were in power, the mandatory retirement age was in the mid-20's, and everyone over 30 was a second-class citizen. Adults were more or less homeless and drug use was rampant among the youngsters due to the pressures of having to succeed so early.
The Star Trek episode "Miri" featured a planet where a virus had killed off all the adults, leaving the children to look after themselves.
Star Trek: Enterprise had an episode dealing with the aftermath of a group of kids being left without adults; unusually, this took place some time after the kids had grown up, and they eventually they had to be left where they were because reintegration would have been nigh-impossible.
One chapter from Stargate Atlantis featured a tribe of kids from age 25 and down. After they reached 25 they committed suicide, but only because they had to keep the population small due to being kept safe by a shield that was slowly becoming smaller. But they didn't know that...
The Farscape episode "Taking The Stone'' had a slightly similar premise, with the members of a hedonistic tribe suffering from cumulative radiation poisoning in their early twenties. Most of them "Take The Stone" at around twenty-two years of age, or else join the outcast "Lost People" wandering the abandoned catacombs for the rest of their lives.
Isaac Asimov's Probe: "Quit It" had a group of teenagers who had discovered a form of mind control that only affected people who had exited puberty. They nearly ran their entire neighborhood into the ground, ordering their parents to have constant block parties, buy boats and expensive cars, etc.
Buffy: "Band Candy" has Ethan Rayne magically infuse chocolate to turn adults into teenagers. The results include Snyder out partying, Giles reverting to a teen rebel and Joyce macking him, all so the Big Bad can allow babies to be sacrificed to a demon.
One of the first episodes of Andromeda had an old High Guard supply station inhabited by the descendants of the original staff, who all died in their twenties or sooner due to radiation poisoning or raids by Magog and Nietzscheans. They were also religious fanatics who worshiped the High Guard and considered a cabinet full of schematics (that they couldn't read) sacred scripture. And when Dylan accidentally unlocked the Nova Bombs they attempted to send suicide Slipfighters to nearby systems.
An interesting case was that of China when the Communists first took over. For a society that had always relied on an extremely rigid age-based hierarchy, suddenly teenage and young adult members of the Communist Party were given life-or-death power over their elder family and community members. Some of them went a little drunk with power for a while. Curiously, many of those who were members of the original revolution as well as the Cultural Revolution are now leading the Chinese government. Considering that they seem obsessed with keeping things orderly, one wonders if it is a case of a Full-Circle Revolution or a result of lessons learned from an anarchic period. YMMV of course.
Parts of the UK for complex reasons. If you take high youth unemployment, a society that runs on rampant consumerism, and then an unarmed man is shot by police, you have a recipe for social disorder. At first it was supposed to be a protest against the shooting and then it became a case of want, take, have.
During the 2011 England riots, entire city centre's became teenage wastelands of looting and ultraviolence for about 4 nights. Not all of them were teenagers but many were under 25, including one 11 year girl arrested for looting.
Given the demographics of prehistoric times, it wasn't unusual for a Neolithic community — especially a newly-established one, or one recently struck by contagion — to consist almost entirely of young adults and small children.
Communities of children who lack sufficient parental nurturing bond with each other instead, leading to the formation of gangs, violence, drug addiction, etc.
Indie tabletop RPG Misspent Youth is all about this, with the PCs being children aged 12 to 17 fighting against the Authority (whatever form it takes for your game). Adults either blithely let the Authority do what it wants, or are on its side. The game actually mentions several of the above examples as inspiration.
In Bliss Stage, alien invaders triggered the Bliss, an condition that's Only Fatal To Adults - and continues to be so: Turning 18 is a death sentence.
This is the entire point of the RPG Kidworld.
The domain of Odaire became a Teenage Wasteland when the evil puppet Maligno killed all the adults in the city, forcing the older children to fend for themselves and their young siblings. Subverted in that their society is actually quite functional and civil, and because they've grown up by the time of the Arthaus 3E products.
The desert domain of Sebua is home to a colony of feral children who don't grow any older, and who live like wild animals.
In the Fighting Fantasy gamebook Starship Traveller, the player can encounter a planet where the kids are in charge because the alien race suffers from extremely rapid and severe senility and dementia as soon as they reach adulthood.
Fallout 3's Little Lamplight is inhabited by children and teens. Upon turning 16, they are shipped off to Big Town.
The game Rule of Rose features a society run by little girls.
The first level of the PC game Sanitarium has a town whose inhabitants are creepy deformed children controlled by an evil being which they refer to as "Mother," who killed all the adults in the town after seeing how cruelly they treated the children.
MMORPGs often have stable group structures, such as guilds in World of Warcraft. Groups with (male) teen leaders and predominantly teen populations become Teenage Wastelands for their members. Whole games may become this, too.
Post-Apocalypse Mobliz in Final Fantasy VI is this. The parents of the town sacrificed themselves to save the children, leaving the oldest remaining children - two 17-year-olds with couple troubles - in charge. Some order is restored when Terra arrives, who becomes a surrogate parent to the lot, but this still leaves the town tremendously age-skewed, especially given that Terra herself is only in her early 20s.
In Homestuck on the planet Alternia, all the adults of the troll species are shipped off to help with the imperial conquest of the galaxy (and prevent them from leading revolts on their home planet), leaving only the young trolls behind.
Parodied in the South Park episode "The Wacky Molestation Adventure", when the kids cause all of the adults in town to be taken away by social services by claiming they were molested. A couple then comes to the town and finds it in ruins and overrun by children. By this time they've split into factions, developed communities and created bizarre religions. Later we find out that it's only been ten days since the parents were arrested.
Jimmy Neutron - when the adults have been abducted by aliens in the movie.
One of these is encountered by Fry and his ex-girlfriend in an episode of Futurama when they believe they're stuck in an After the End scenario. Later on they learn that LA in the future is just a hellhole...or more of one, anyway.
An episode of Martin Mystery had a town were adults were sent to the cyberspaceto an evil alien that uses the energy of others to get out of his web prison.
In The Fairly Odd Parents hour long musical special School's Out!, Timmy wishes that kids ruled the world, and is elected President. As usual, this starts out really cool but culminates with dystopian catastrophe. It wasn't the kids' fault, though — it was those freaking pixies.
