Mental Speed
Psychological tests reveal that the brain is at its fastest at the age of 18-20 yrs, maximum upto 25 yrs. After that mental speed declines - the sharper the brain, the slower rate of fall.
I think this kind of statistical information only makes sense under the condition that we actually use all our potential for the game, which is far from being true. I'm pretty sure that if I keep practicing harder and harder, I'll get faster and faster with my calculations up to, I don't know, 50 or something, simply because my current calculation abilities and the amount of work I put in the game are way lower than my (any human's) potential.
let's mention some numbers for clarity's sake:
take a 18-20 year old who has worked 1 hour everyday for the past 5 years.
then let this person grow older and older, but this time let him work 8 hours a day until age 50. I'd bet the 50 year old guy would beat him easily in terms of calculation speed.
I'm not arguing against the numbers you mention, just trying to state they need to be interpreted in relation to potential and practice.
I think chess is the second greatest puzzle... after the universe
well, that's still subject to discussion.