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.
