Possibly of interest:
"... the NM title is an honor that only one percent of USCF members attain. ..." - IM John Donaldson (2015)
http://www.jeremysilman.com/shop/pc/Reaching-the-Top-77p3905.htm
What It Takes to Become a Chess Master by Andrew Soltis
"... going from good at tactics to great at tactics ... doesn't translate into much greater strength. ... You need a relatively good memory to reach average strength. But a much better memory isn't going to make you a master. ... there's a powerful law of diminishing returns in chess calculation, ... Your rating may have been steadily rising when suddenly it stops. ... One explanation for the wall is that most players got to where they are by learning how to not lose. ... Mastering chess ... requires a new set of skills and traits. ... Many of these attributes are kinds of know-how, such as understanding when to change the pawn structure or what a positionally won game looks like and how to deal with it. Some are habits, like always looking for targets. Others are refined senses, like recognizing a critical middlegame moment or feeling when time is on your side and when it isn't. ..." - GM Andrew Soltis (2012)
https://web.archive.org/web/20140708093409/http://www.chesscafe.com/text/review857.pdf
100 Chess Master Trade Secrets by Andrew Soltis
https://web.archive.org/web/20140708094523/http://www.chesscafe.com/text/review916.pdf
Reaching the Top?! by Peter Kurzdorfer
"... On the one hand, your play needs to be purposeful much of the time; the ability to navigate through many different types of positions needs to be yours; your ability to calculate variations and find candidate moves needs to be present in at least an embryonic stage. On the other hand, it will be heart-warming and perhaps inspiring to realize that you do not need to give up blunders or misconceptions or a poor memory or sloppy calculating habits; that you do not need to know all the latest opening variations, or even know what they are called. You do not have to memorize hundreds of endgame positions or instantly recognize the proper procedure in a variety of pawn structures.
[To play at a master level consistently] is not an easy task, to be sure ..., but it is a possible one. ..." - NM Peter Kurzdorfer (2015)
http://www.thechessmind.net/blog/2015/11/16/book-notice-kurzdorfers-reaching-the-top.html
http://www.jeremysilman.com/shop/pc/Reaching-the-Top-77p3905.htm
"Yes, you can easily become a master. All you need to do is some serious, focused work on your play.
That 'chess is 99% tactics and blah-blah' thing is crap. Chess is several things (opening, endgame, middlegame strategy, positional play, tactics, psychology, time management...) which should be treated properly as a whole. getting just one element of lay and working exclusively on it is of very doubtful value, and at worst it may well turn out being a waste of time." - IM pfren (August 21, 2017)
https://www.chess.com/article/view/can-anyone-be-an-im-or-gm
https://www.chess.com/article/view/don-t-worry-about-your-rating
https://www.chess.com/article/view/am-i-too-old-for-chess
https://www.chess.com/article/view/how-can-older-players-improve
I'm not a neuroscientist, I don't know exactly why.
I've known adults who have multiple titled coaches (yes, that's very expensive for them) and play in tournaments almost every week.
They work hard, and they improve a lot. Some make it to around a 1800-2000, but then they seem to stop improving. I don't know why.
I do know that 1800-2000 is very far away from an IM title.
I also know if you're a coach who can turn adult beginners into titled players you'll be able to charge quite the price... because you'll be the only one in the world who can do so