why starting Chess young is so important?

If you're good enough to start young then you do it. I don't think it's that people who start young are good, I think it's people who are good start young.

I just started not to long ago.

The juvenile brain is more malleable.
If you start later, then you can progress faster, but your ceiling is lower.

Younger children do seem to absorb information like a sponge, but I imagine that another reason they succeed more than older beginners to chess is because chess requires a lot of time to invest into improving and also requires a ton of determination; children typically have both!

Once you get older, typically other responsibilities such as school, work, family etc. take up a lot of your free time. Similarly, someone older might be more likely to have more interests and indirectly less time for chess. 

As @tygxc mentions, "The juvenile brain is more malleable" but I don't know if I agree with their part after that with the faster progress and ceiling being lower. Let us not forget that Mikhail Tal reached his 2700+ rating peak when he was 44 years old! Granted, he had been a GM for a long time (18 years old when Tal became a GM I think? Someone fact-check that one xD), but he simply kept improving as he got older and older. It is clear he never stopped improving his chess game and his love for chess kept up - shame Tal passed away earlier in life (no doubt his drinking, chain smoking and partying didn't help).

Anyhow, 44 years old and reaching a peak rating displays how children aren't the only ones who can reach amazing heights.

#5
Tal started at age 7. Of course a player can still become better after age 40. However, if the same person Tal has started at age 17, or 27, or 37, then he would have reached a lower peak.

Yeah

Thanks for checking when Tal started chess and yeah perhaps he wouldn't have gotten as highly rated if he had began chess later in life.

Of course, I think age isn't much of a factor at all in reaching 1500+ chess.com rating. I choose 1500 just because that is roughly 90 percentile globally. If you are playing chess as a hobby, or personal challenge, then age likely won't stop you from improving and enjoying the game; but if we are specifically talking about the world elite GMs and especially super GMs of 2700+ rating, then yeah beginning later in life makes the unlikely feat even more unlikely.

#8
1500+ is nothing.
For 1500+ it is enough to check the intended move is no blunder before you play it.

Lasker said 200 h are enough to play on par with a master, provided the 200 h are spent the right way:
"Having spent 200 hours on the above, the young player, even if he possesses no special talent for chess, is likely to be among those two or three thousand chessplayers [who play on a par with a master]. There are, however, a quarter of a million chessplayers who annually spend no fewer than 200 hours on chess without making any progress."

I admit it has been a long time since I've been 1500, but it isn't "nothing" to reach that level as most players don't get even that high. Yeah, I know 1500 is like nothing to regular tournament players like we probably are, but that doesn't mean 1500 took "nothing" to reach. 

I still remember how happy I was when I first crossed the 1500 barrier years ago

The fact that 1500 feels like nothing compared to the higher levels doesn't put down 1500, but instead reveals just how much there is to learn in chess and how incrementally tougher it becomes to increase rating at higher levels.

#11
Just do not hang pieces and pawns and you are 1500.
Many, many study openings, watch videos, play bullet and blitz and still hang pieces and pawns and stay below 1500.

It can't simply be that simple.

There are those who do not know how to checkmate in the endgame too!

It's like learning a language when you are young. Doing so later in life isn't the same at all.

This is nonsense. 1500 isn't "nothing". If the average player starting studying Chess it would take them quite awhile to reach that level. 

It's simple to explain.  Compare it to a sponge. An adults brain is a wet song very full of information yet not able to pick up much more. Children are a dry sponge, not super knowledgeable but able to soak up Information and more willing to soak it in. A child takes information in much easier than an adult does. Some adults like the youtuber mark rober are always dry, always willing to soak up more. Adults use the left side of their brain more than anything else, which is their the Information and more serious side. That's the one that instinctively tells you every morning to go brush your teeth. Children use their right side more and if the left side is serious the right side is creative. If you are an artist that's which side tells you to get an idea in your head. The right side is what helps you make non instinctive or creative decisions. Same goes for chess. Adults generally peak and are unable to listen to what to do after that. Children are much easier to soak things up. Hopefully that was a good lesson on how your brain works for younger and older people.

chess brain tends to peak at around 35 or so, but that alone doesnt explain why older players getting to GM is so rare.  There probably is a brain plasticity advantage for getting really good at young but is not absolutely necessary.

i think the main reason is that childhood and adolescence is the perfect years where excess of time can be capitalized on, especially if they forego a rigorous learning curriculum. Think for example how many hours a teenager can spend on one game (i used to play a bit and a good story rpg can easily take 80-100 hours). that combination of time (and energy) without competing responsibilities makes a huge difference, and chess has certain social components where you are even more motivated to get strong quickly. You also normally dont have to worry about funding your improvement either.

Didn't know somebody already posted about the brain idea just consider my post an addition sorry. I can assure you I didn't copy thus comment.