I have personally noticed some drop in language acquisition skills, but I'm learning Spanish at 35 and I wouldn't say it's significantly harder than learning English at 12 (apart from the fact that English is everywhere around me all the time, but Spanish is very hard to come by).
Maybe this will help you?
http://www.jneurosci.org/content/36/3/755
http://pandora.cii.wwu.edu/vajda/ling201/test4materials/seco...
https://www.sciencedaily.com/releases/2012/03/120328172212.h...
https://www.sciencedirect.com/science/article/pii/S0093934X0...
https://books.google.co.uk/books?id=5vvY2BDWZjEC&pg=PA251&dq...
Pinker's The Language Instict.
https://en.m.wikipedia.org/wiki/The_Language_Instinct
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2947444/
Noam Chomsky, the world's most popular linguistic https://study.com/academy/lesson/noam-chomsky-on-language-th...
15 is already considered very late to start serious study in chess. You simply do not develop the unconscious mastery the blink like ability to play a good move instantly.
It is not the lack of time in older players either. Tales abound of players of independent means dedicating themselves to chess and getting nowhere near master level.
By reverse, no chess player started at 25 and made Grandmaster.
A huge outlier is https://en.wikipedia.org/wiki/John_K._Shaw who was a club level player at 19.
Quick search turns up this list: https://www.quora.com/Is-there-any-famous-chess-player-or-gr...
Chigorin is from a different era, but he didn’t begin playing “seriously” until 24, and was a 2 time world championship challenger.
Getting to master (2200 rating) level play is ultimately just an investment of time and money. For better or worse, the “consensus” is that master level play is not due to some kind of innate ability. Drilling the pattern recognition inherent in all phases of play is the key determinant for reaching ~2200 rating. Do a cursory Google search, and you’ll turn up hundreds of threads about this topic.
Becoming an international master (IM) or grandmaster (GM) requires navigating FIDE’s, the world’s chess governing body’s, arcane rules around norms, or results, for the GM or IM title. I’m by no means an expert on the scientific piece of this, but I’d posit that the number of adults who have the financial means and time to travel across the United States or world to find tournaments that will earn them a norm is negligible.
When you’re young and your parents are paying the bills, chess can seem fun and rewarding. When it’s you personally who is paying the bills for tournament registration, hotels, and planes, it’s a lot less fun.
My bias: I am a FIDE master who skipped chess study for a year when I was a teenager. (insert Brando quote from On the Waterfront here).
Notice that the main examples that would disprove my premise are from 19th century. You'd need chess historian Winters to sort the claims but Chigorin's "seriously" is something to debate.
Why aren't modern influx of adult players making masters? Plenty of adults playing in tournaments all around the world spending thousands on travel and equipment and training. Most of us are having fun but the improvement is not there.
Also GM Klovans was a master level player in his teens and GM strength player as a young adult. Source: knew him personally and cherish the only draw I had against him.
The few modern day examples started in late teeens and fit within my narrative.
If there is a "consensus" that becoming a master does not require some innate ability then where are those masters? Show me a master level player who started at age 30.
The chess.com megathread on this topic came up with an expert who achieved his title after starting at age 30 and that really is an outstanding accomplishment and a huge outlier. If "consensus" is correct there should be more numerous examples.
Pretty much all adult players have hit a serious wall this you can see by looking at rating charts.
The only exceptions are returnees who dedicate themselves to chess full time and even then the gains are miniscule 50-100 rating points at best and then they hit the wall and that's that.
Thus I take serious issue with your presumption that "Getting to master (2200 rating) level play is ultimately just an investment of time and money."
My thesis that chess skill/rating = talent * early start * dedicated study . You need all 3, if one is missing your chess skill will be seriously hampered.
My own bias was as a competitive junior, who had several of my peers reach 2000-2100 before they were in high school (some as early as age 10). Several of these kids had languished for a few years with mediocre progress, before taking on serious study habits and effective coaching. Almost to a person, they stopped playing in high school because of academic/athletic/college time demands.
Again, maybe there's some biological/scientific reason why people late to the game of chess cannot perform the pattern recall that is central to chess. But, I'm still inclined to think that the demands of "real life" are a key reason why players starting at an older age are unlikely to reach master strength. Yes, there are many adults who devote time to the game. But, how many are able to devote the same amount of time and energy that a schoolkid is?