I was thinking about something I had read about...is it theta waves? Something that happens to kids' brains around puberty? Something that differentiates a child's brain from an adult's brain. I'd love for it to be really specific...like happens on a 12th bday or something, but that's probably unrealistic. I'll have to make that part up. (thank god I like writing fiction!)

Like I said, it just needs to be semi-plausible, but I would appreciate a few pointers or ideas for what kinds of things to research, since it's been far too many years since I last looked at any neuroscience papers...

I think you will run into plausibilty issues without this 'change' being somehow linked to puberty. You may need to think about the fear of menstruation/puberty trope that is common in sci-fi.

Edit: by 'think about' I probably mean 'avoid'. Once recognised, you will see the 'mentruation/puberty is evil, dangerous or bad' theme is overused, especially in sci-fi.

Having said that, did you know that there are sounds that kids can hear that adults cannot?

See hear (pun intended).

Perhaps you can apply that somehow.

There are social media platforms that are used mainly by teens and not adults and vice versa.

Does it have to be physical?

By way of more information, my MC is going to be (this is my nano project I'm planning so she hasn't done anything yet!) on a spaceship where all the sudden all the >12 year olds conk out (sleep-like) and she has to race against time because her 12th bday is coming up and she is the only one who can figure out how to pilot and land the ship they're in. Along the way all sorts of mayhem will take place, of course.

I'm loosely inspired by both the book Brain Wave, by Poul Anderson (premise is that the earth suddenly leaves a "dampening field" that has been hampering our IQ for eons, and all the sudden everyone is much much much much smarter), that I read a month ago or so, and a book I read as a young girl over and over that is set in my home town called The Girl Who Owned a City (premise is that all >12 year olds in that story have died and now the kids are in charge. My book is taking a more soft-shoed approach...the grownups are there, but incapacitated...in a way that is humorous because the kids have to keep climbing over passed-out adults.)

You know how sometimes the best ideas come from a collision of two ideas? So I have the basic idea of a bunch of kids in charge of a space ship, but I just need to have that semi-plausible explanation for why the grownups are conked out. I feel like it's important to have the MC's birthday as a hard and fast deadline that is approaching quickly, she has to solve the puzzle before she conks out herself.

I don't think a rite-of-passage ritual (ala the eating hot peppers thing you suggested, extrin) will work because she's too smart to go along with a ritual just because that's the way it's usually done, particularly if she's in a crisis already and she's seen other 12 year olds during this crisis conk out after eating the peppers or drinking the 12 year old punch or what have you.

FWIW, I wasn't planning to use puberty as the demarcation because it's too broad a range. I want her to have a ticking time bomb both because of external factors (spaceship will crash if she doesn't figure something out) and her own factors (and soon because she's going to be 12 any day now!)

But since this is scifi, what about having some genetic or drug induced alteration of humans done by doctors to protect young children from getting diseases, but these alterations also inhibit puberty. So the smart scientists programed the genes or drugs to "turn off" at age 12. So this alteration protects everyone under the age of twelve from the disease inducing comma, and the mc knows that her protected days are numbered.

Just an idea.