Why do child prodigies seem to barely improve after the age of 12?

Sometimes I see a 20-25 year old guy who's an FM and has a 2300 rating, and I think to myself "I wonder how good they were when they were a kid". Then I do some research and realise that they were already above 2000 rating when they were 12 years old! It's like they just skyrocketed to expert level at a very young age, then barely showed improvement after that.

Other times I'll see a 20-25 year old guy who's about 2000 rating and what do you know: they were already 1700 rating when they were 12 years old! Once again, they just popped up out of nowhere with a very strong understanding of tactics, endgames and positional awareness... then pretty much just stagnated for the next 10 years after that.

What's with this? Why do people seem to show massive leaps of improvement from the age of about 8 until 12 years old, then they suddenly slow down after that, as if their brain somehow stopped growing after the age of 12? It's so counterintuitive.

School.

Let's all focus our mental energy and try to find s. th. that might be more interesting for s. o. being between the age from 13 to 20 rather than moving around wodden pieces  on a board game...

If you play golf, to get your round from +72 to +36 will take a bit of work. From +36 to +18 much more (even though it's half a step). And from +18 to +9  many more years. To get to 0 possibly a lifetime. If you have any time left from that goal post, getting to -18 is likely impossible.

The steps aren't linear, it's exponentially harder to improve as you go. Youngins have an incredible neuroplasticity (according to my vast YouTube Ted talk watching, and no actual neuroscience knowledge) and can learn and process incredibly fast. What they don't have is experience. As they gain experience they start losing processing power... In the videogame world now, right around 16 is when you have the fastest reflexes and sufficient breath of experience to be at your individual peak. I don't know when that number is for chess, but it's likely mid to late 20's (and reducing as reliance on computers can ramp up experience much faster than in the past) based on world champion contenders.

If you were extremely talented at something, wouldn't you rather focus your efforts on that, rather than just learn some other skill that you're only mediocre at, so that you can live a "normal" life just like everyone else? That seems like such a waste of talent to me.

When we are kids, our attention and focus quality is much better than our adult age . As you can see, there are a lot of new experiences and problems/new skills/managing people and income and etc stuffs come in to our minds and divides our priorities. Focusing on so many things at the same time impacts our memories and attention span along with creativity as well . Also, the competition above FM level becomes  harder too .

How much money do you think Vincent Keymer's parents are paying Peter Leko to coach him and travel to Germany for the Grenke Classic?  How many kids who have potential to be me much better have that kind of support?  

Obviously going from 800-1200 is way, way easier than going from 2400 to 2500. Its not even close, the players are better so every step you take is going to require finer and finer edges and subtler understanding which takes way more study and work. 

Even in my own crappy chess career, I can see how much easier it was to progress the first few hundred ranking points than it has been ever since.  Players get better, the knowledge and theory needed gets way more intense as you climb.  Shouldn't surprise you, I'm surprised that you're surprised! 

Puberty.

Also, brain plasticity is much higher in children than in young adults.

boredom. chess isn’t all it’s cracked up to be. it used to be a sport but apparently now it’s only a game, since chesscom is the new ruling body.

Me when I was 8 - 1550 

Now, when I'm 14 - 1650

Idk what happened.

A thousand Blitz games?

You don't think 2000 to 2300 is a significant improvement? 

EXACTLY. a 300+ rating is a very large improvment.

The enjoyment of italics. 

300 points is massive at that rating. As a teenager, I would be content with gaining 200 points from 1800 to 2000 and being there for the rest of my life.

Anyway, to answer the above question,

1. School, as it is becoming more demanding these days. Middle/High school has turned from a house of learning to a House of Pain in recent days.

2. Spending time on other hobbies. For me, that is trolling.