In this world, physical age has little legal relevance. Of course, people still age, and birthdays are usually celebrated, but things like when you start school, when you get to drive, or vote, or drink alcohol and all those things, they are basically independent from your age. What matters is which maturity license you have.

There are eight maturity levels: Infant, Toddler, Child, Teen, Young Adult, Adult, Middle Age, and Elderly. Which license you have determines which kinds of privileges and restrictions you have. Infants and Toddlers have no autonomy at all, and always have to be looked over by their parents/guardians. Children and Teens still need their parents, but have a few more freedoms. And so on. On top of that, only Children and up are allowed to use toilets. Toddlers either wear diapers or pull-ups, or have to use plastic potties, and Infants need to use diapers. Since it can't be determined which maturity level a person has just by looking at them, everyone wears a special collar that denotes their maturity level: White for Infants, red for Toddlers, orange for Children, yellow for Teens, green for Young Adults, blue for Adults, purple for Middle Age, and black for Elderly. When you change maturity level, you're given a new collar and your old one is discarded. It is impossible to take a collar off, except with very special tools. And even if you do, people without one are automatically classified as infants.

To get a higher maturity license, you have to take a test. However, when you take those tests is not determined by age. While it is usual to take a test for a certain level at a certain age range, it is totally possible (if unlikely) for a twelve year old to be a Young Adult, or for a sixteen year old to be a Child. Infants, Toddlers, and Children need to take their tests when their parents/guardians say so, and only then. Teens have a bit more say in when to take their test, and they can even take their test without parental permission, however, if the parents/guardians veto against it, they can still be demoted back to Teen, unless they excelled at their test. In any case, tests can only be taken in the correct order. So, a Child could not take an Adult test, for example. Also, you can only take a test for one level if you spent at least a year as the previous level.

The maturity levels also play a part in the legal system of this world. Not just because, of course, your maturity license determines what you can and cannot be tried for, but also because being temporarily demoted to one of the early three maturity levels is one of the possible sentences judges can give you.

There is of course also abuse in this system. Like parents who don't let their offspring take the test, so they remain Children, Toddlers, or even Infants forever. Or cases in which a person adopts a Teen, Child, or Toddler, or gets custody over them in a divorce, and then appeals to a court that they should be demoted because now that they're the new guardian, they think their children aren't ready for the higher maturity levels. It has also happened that people past Young Adult stage have been demoted back to Child, Toddler, or Infant, due to it turning out they actually failed one of their tests, or even due to a bureaucratic error.

Since Infants can theoretically come in all shapes and sizes in this world, it is pretty easy to find things like diapers, high chairs, changing tables, cribs and so on in all sizes. Also, it's way more common to find explicit diaper changing rooms in public places, with differently-sized changing tables, than it is to just have a family restroom with a single changing table.

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I would like to play as someone who, for whatever reason, gets demoted to toddler or infant status. I would also like the setting to be as grounded and "realistic" as possible. There shouldn't be 30 year old infants and 12 year old adults around every corner. 20 year old toddlers should be annoyed to be babysat by their younger adult siblings, etc. And the people around me should need some time to adjust to my new status.

We can discuss details in DMs, but here's an idea to get started. But feel free to propose your own scenario:

I'm a 30 year old Young Adult and you're my 25 year old Adult girlfriend. We started off as just roommates and haven't been dating for long. We haven't even slept with each other yet. One day, we get a visit from someone from the Ministry for Maturity, who explains the following situation to us:

The thing is, I have always been a bit behind. I didn't become a Toddler until I was 5, and became a Child at age 12. I didn't become a Teen until age 20, and only passed my Young Adult test at 28. And with "late bloomers" like me, there's a sort of "probation" thing when it comes to the Adult test. Usually, when a Young Adult fails the test, no matter how bad they fail, it's not that big of a deal, they can retake the test a year later. But with late bloomers, if the test is failed, they actually get demoted to a lower Maturity Level, depending on how badly they failed.

I'm an Infant again.

This news shocks us, but you offer to become my caretaker, so we can at least stay together. The Ministry is okay with that, and will supply anything to get you started in caring for an Infant my age, but they also warn you that they will keep an eye on you, with announced, unannounced, and secret controls, to make sure you treat me in an appropriate way. If they think you don't, I will not only be given to a more responsible Adult, but you risk demotion of your level as well.

And so begins my new life. And as it goes on, you will start seeing me more and more as a little kid. Will you even let me graduate to higher levels? And if so, how far?