How would a world occupied by just teenagers occur?

Question

My story is set in a post apocalyptic setting where there are only teenagers and children. Also, cities are overrun with knee high grass and large lakes. Animals have evolved and have gotten more dangerous. A literal concrete jungle. Technology is very primitive again. Some teenagers though have mutations with give them powers that are dangerous to everyone even themselves. I'm thinking diseases or something.

Answer 1

from OP: /Some teenagers though have mutations with give them powers that are dangerous to everyone even themselves. /

The older you get, the more powerful you are. And the more dangerous you become.

The powers grow. Just when a teen has got the hang of it, the volume goes up and new powers sprout. These powers are tempting to use but risky, because your abilities will have increased since the last time you used them.

Teenagers wind up dying in accidents, or during a dream. It can be messy. Some teens leave to protect the people they care about from themselves. There may be some adults roaming the wild lands, alone and mentally unhinged from the godlike powers raging inside them.

Answer 2

Aliens want pets

Aliens want all sorts of exotic pets. They do, however, have an Animal Rights Society, that guarantees the treatment of all semi-sentient creatures.

Some evil Alien smugglers want to obtain these pets illegally, and to get away with their evil schemes, they biologically engineer a 'plague' on Earth that guarantees any human over a certain age dies. In the ensuing panic, no one will ever notice if a few mature adult humans are abducted. As to why they don't take children, they have enough adults to choose good ones from, and are constantly monitoring the situation. Why train them when they'll train themselves?

To make the pets even more useful, they have injected humanity with superpowers, and they will be watching for those who end up manifesting exotic powers...

Who knows, though, whether these evil aliens will have it their way forever? After all, teens will always rebel.

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A more serious answer (but this works with the alien idea too!):

Superhuman Disease

A disease has manifested, which causes humans to exhibit super-powers. Humans, upon receiving the disease, will find that their appendix's potential has been unlocked, and is now capable of passively generating huge amounts of energy, and they can use this energy to cause various phenomenon (How this grants them superpowers can be explored in other worldbuilding answers).

The issue here, is that with age, the organ's ability to control the energy deteriorates as the organ starts to lose function. As a person passes from their teens to adulthood, their ability to control the energy slowly decreases, until there is no longer any way to control the energy anymore.

At this point, the body begins to break down, unable to bear the massive amounts of energy. Thus, there are very little humans who make it past their teens.

The only way to survive is to grasp the methods of controlling the energy within when their organ is fully functional, and this is much easier for people in the their childhood periods. These people with excellent control will have relatively longer lifespans, being able to handle more energy than the non-ability users. Someday, however, they will also be unable to handle the energy within, and their body will start to break down as well.

Answer 3

A shock shortage of resources in a demographic that has a significant fraction of people over 20 means that, realistically, on a small proportion of the population can survive long term. In such a case, a society could decide to sacrifice those older than a certain age for the survival of those younger. It's conceivable that the stress is so bad that one has to make that certain age very low, and that there would be a high enough fraction of those left in their late teens that society judges that these late teen people could afford the leadership and nurturing needed to keep society going.

Of course, not everyone is going to agree to this plan, so you would need to flesh the details out. Some older people would be compelled if the culture were extremely society-focussed so that a vast majority of the older generation would decide for the altruistic alternative. You might have an extremely high proportion of people in the demographic who have below-20 aged children.

Indeed, the above idea is not original, something very like it is the basis of the 1967 novel "Logan's Run", by William F. Nolan and George Clayton Johnson and the 1976 Film and TV Series grounded on this book.

Answer 4

A more virulent for of coronavirus has swept over the world. In children and teenagers the disease is almost asymptomatic, while in adults over 20 years of age is has a very high fatality rate.

This world, though, would have a rapidly declining population (assuming there's no modern level medical care) because teenagers can't reproduce fast enough to compensate for the population loss.