Age

Possibly of interest:
https://www.chess.com/article/view/can-anyone-be-an-im-or-gm
What It Takes to Become a Chess Master by Andrew Soltis
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
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
What It Takes to Become a Grandmaster by GM Andrew Soltis
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/blog/smurfo/book-review-insanity-passion-and-addiction
http://www.nytimes.com/1988/09/26/books/books-of-the-times-when-the-child-chess-genius-becomes-the-pawn.html
https://www.forbes.com/sites/alexknapp/2017/05/05/making-a-living-in-chess-is-tough-but-the-internet-is-making-it-easier/#4284e4814850
https://www.chess.com/news/view/is-there-good-money-in-chess-1838
"... Many aspiring young chess players dream of one day becoming a grandmaster and a professional. ... But ... a profession must bring in at least a certain regular income even if one is not too demanding. ... The usual prize money in Open tournaments is meagre. ... The higher the prizes, the greater the competition. ... With a possibly not very high and irregular income for several decades the amount of money one can save for old age remains really modest. ... Anyone who wants to reach his maximum must concentrate totally on chess. That involves important compromises with or giving up on his education. ... it is a question of personal life planning and when deciding it is necessary to be fully conscious of the various possibilities, limitations and risks. ... a future professional must really love chess and ... be prepared to work very hard for it. ... It is all too frequent that a wrong evaluation is made of what a talented player can achieve. ... Most players have the potential for a certain level; once they have reached it they can only make further progress with a great effort. ... anyone who is unlikely to attain a high playing strength should on no account turn professional. ... Anyone who does not meet these top criteria can only try to earn his living with public appearances, chess publishing or activity as a trainer. But there is a lack of offers and these are not particularly well paid. For jobs which involve appearing in public, moreover, certain non-chess qualities are required. ... a relevant 'stage presence' and required sociability. ... All these jobs and existences, moreover, have hanging above them the sword of Damocles of general economic conditions. ... around [age] 40 chess players ... find that their performances are noticeably tailing off. ..." - from a 12 page chapter on becoming a chess professional in the book, Luther's Chess Reformation by GM Thomas Luther (2016)
http://www.qualitychess.co.uk/ebooks/LuthersChessReformation-excerpt.pdf

It has not happened in modern times. Like football, mathematics, piano, dance, etc. it's very hard to begin learning at age 20 and become highly skilled. It's much easier to learn such things when you are very young. Some studies suggest that changes take place in the brain around age 12 that make abstract things much more difficult to understand if you haven't learned their key points by age 12. My experience agrees with that I had a strong interest in chemistry and math from a very early age and a very high IQ and became a master chemist. I learned serious basketball in my early teens and barely made my school team. I began studying piano in my 50's and cannot play as well as the 12 year-olds that my elite teacher also teaches. It took me YEARS to qualify for a major student piano recital.
I got serious in chess in my late 20's, eventually reached a 2116 USCF oorrespondence rating, had excellent results coaching a high school team, but OTB in fast games I have never been anything close to great.
Perhaps it depends on the sort of professional one is hoping to be.
"... I’ve known many very low-rated chess teachers who are absolutely excellent. Teaching is a skill, and even if you’re rated 1500, you might be just what the doctor ordered for children or beginners of any age. ... let’s not leap into teaching quite yet. ..." - IM Jeremy Silman (2016)
https://www.chess.com/article/view/can-anyone-be-an-im-or-gm
Still sounds like a poor and uncertain sort of life.
"... a profession must bring in at least a certain regular income even if one is not too demanding. ... With a possibly not very high and irregular income for several decades the amount of money one can save for old age remains really modest. ... For jobs which involve appearing in public, moreover, certain non-chess qualities are required. ... a relevant 'stage presence' and required sociability. ... All these jobs and existences, moreover, have hanging above them the sword of Damocles of general economic conditions. ..." - GM Thomas Luther (2016)