There is definitely the aspect of talent. I've seen (and arbitrated) players who began at an early age, had the most effective training and became completely average or even stagnated. And I've also interacted with those who heightened their chess skills in a really short time and are among the highest rated players.
Point well taken, but I'm still not convinced that the variability you describe couldn't be accounted for by things like differences in diet (which can greatly affect cognitive ability), or differing circumstances at home (ie. abusive homes, or parents who are neglective vs caring and nurturing) and/or outside the home (like various forms of stress when kids live in dangerous neighborhoods where their peers and adults get assaulted or murdered on a regular basis), parents who push their kids to achieve vs ones who don't care how or what their kids do, etc...
Picking out the effects of nature vs nurture in a random chess playing kid is not easy, and I wouldn't be so quick to attribute to innate talent that which could be explained by environment.
What is clear, however, and something which you did not seem to deny in your own response, is that playing a lot of chess when you're young is critical to overall chess performance. Without that even "intelligent" and very capable adults who started playing too late are just not going to catch up to capable kids who played a lot of chess early.
Yes, I may have assumed that all other variables are held constant, which is usually not the case. Although on your other point, I feel as though your argument sounds more like the 10,000 Hour rule popularised by Malcom Gladwell which I don't 100% agree with.
But I don't believe an adult who started playing chess late and plays 10,000 hours will be anywhere near as good as a child who started playing chess early and played 10,000 hours (all other variables held constant).
Also, I have no confidence in a 10,000 hour (or any other fixed number of hours) figure. The hours it takes to attain a certain skill in chess will vary from person to person, depending on all sorts of intrinsic and extrinsic factors, including their training regimen, the skill of their teachers, and many of the factors I listed in my earlier post.
