Never Land

What happens when Not Allowed to Grow Up affects an entire setting. This is where an entire area, often an Adventure Town, is entirely populated and governed by children.

They might actually be children, or they might actually be Really Seven Hundred Years Old, but they will always at least look like little kids.

The trope name is something of a misnomer, since in J.M. Barrie's Peter Pan, being on Neverland didn't actually stop anyone from growing older, though it works that way in many adaptations.

Sister Trope to Lady Land, where most of the inhabitants are adults, just monogender.

Film

 * In The Santa Clause, aside from Santa/Scott himself the North Pole is like this. The elves are ancient beings but the eldest of them looks no older than fifteen or sixteen tops and most of the others look much younger.

Literature

 * Cittagazze in His Dark Materials. The Spectres suck the soul out of anyone who has hit puberty.
 * In Everlost by Neal Shusterman, the titular Everlost is populated by ghost children who are Really Seven Hundred Years Old. One of the oldest-looking children was fifteen at the time of death.

Live Action TV

 * The Reality Show Kid Nation was based off this trope.
 * The Star Trek episode "Miri" contains a fairly dark example: A planet of long-lived, unaging children who sicken and die upon reaching long-delayed adolescence.
 * The same principle was the basis for the New Zealand After the End kids drama The Tribe
 * The Stargate Atlantis episode "Childhood's End" features a set of villages populated entirely by children (and young adults) because they commit ritual suicide upon turning 24.
 * Naturally, Sy Fy's miniseries Neverland, which serves as an Origin Story for Peter Pan, and "explains" that no one ages in Neverland because it's on a planet in the center of the universe, so time stands still for anyone there.

Video Games

 * The Kokiri Forest area in The Legend of Zelda: Ocarina of Time.
 * Interestingly, it's never revealed whether the reason is that the Kokiri themselves are immortal, if it comes from all the fairies that congregate there, or if it's because of the mysterious protection of the Deku Tree.
 * Youngtown in MOTHER 1, due to Alien Abduction of all of the adults.
 * Little Lamplight in Fallout 3 has been populated entirely by children for nearly two hundred years. It's not clear how they replenish their population, but citizens are banished to Big Town on their eighteenth birthdays.

Web Comics

 * Tales of the Questor features this with the elves - due to a goof by a now-reviled elven king (who wanted his people to be able not to die of old age and apparently went to a Literal Genie for his wish) elves are perpetually young - but not long-lived. They only live to about twenty or so, and then die of 'old age'. The result is that most of the elven territories are inhabited by half-wild children. To make things even worse, it's implied that, as a result, their civilization has gone from being able to match the humans or even the Sidhe to something out of Lord of the Flies. Half-elves, although longer-lived, still have much briefer lifespans than humans or Raccoonan - they only get anywhere from ten to twenty years more. There is allegedly an artifact out there somewhere that can fix the problem, but it's missing so it looks like our hero Quentyn has another item to keep an eye out for in his grand quest.

Western Animation

 * Codename: Kids Next Door: During a Chase Scene, Number 1 crashes and wakes up on a Utopian island inhabited entirely by children. He soon learns he's actually inside a Lotus Eater Machine.