It all began with the dream of a madman. An insane wizard who was motivated by reasons only comprehensible to him. In the accumulation of their life time of effort, they single handed cast a slew of spells around the world, perhaps invoking some forgotten ancient gods and calling upon their might.
Every infant, toddler, child, teenager, middle aged, and elderly, all spontaneously progressed or regressed to the age of eighteen. With an education imprinted directly in their brains on how to be functional adults in society. Any other pregnancies that followed would have the same thing happen at the moment of birth.
Everything was frozen in the present. The present has remained for over fifty years.
But there are some things that need to be learned through experience in one's formative years, that no longer exist. Empathy, kindness, sympathy, pity, mercy, there are learned traits, traits which that phase in our lives no longer exists to learn.
In half a century, sociopathry has sky rocketed. Charities have dried up. Concepts like family, and the bonds and endearment that come with it, have also been wiped away. Innocence no longer exists. And nothing is considered cheaper, than human life. Magic is wielded like a caveman's club.
This is where you come in... one old nanny, part of the old guard, who has watched the world turn ugly. Whatever gods are listening, they've seen fit to now answer your prayers. If in their own entertaining way.
You hadn't expected to be struck by lightning on a clear blue day, and became a anthro kangaroo complete with a pouch on proud display. You hadn't expected your home to be now bigger on the inside than the outside, able to expand infinitely as needed. And you hadn't expected to be able to create replicas of yourself, all with your motivations and goals, to keep the future workload from driving you mad.
And you certainly didn't expect to be granted by divine mandate the power to turn these eternal eighteen year olds who have ruled the planet (most planning to build a spaceship once this planet was used up), into furry toddlers, granting them the childhood they were supposed to have.
The war between adulthood and childhood has begun.