There are pictures of Reshevsky playing simuls at a very young age -- I think there is some controversy over his exact age since the younger he was supposed to be, the more interesting. He also, even as an adult, was not very tall so I suspect SR looked very young at a given age.
His opponents were sometimes grey beards and I think he played 20 cadets and won almost every game at West Point -- even though Elo ratings did not exist and most players I suspect had not much access to top competition or even that many books in the early 1920s, I would suggest that the simul at the US military academy must imply near master strength.
I think Morphy's games when he was under 12 probably show him to be a master quite young and same thing with Capablanca. Both of these players had access to very good competition that few people in the 19th century had although neither lived in London or Paris which I think were top cities for chess competition then.
In modern times where there were ratings, do we know the fastest, starting from learning the moves to 2200 Elo (FIDE) at any age the shortest time to master? I suspect it might be a kid but I also think a teenager starting from scratch would reach master sooner than say a six year old also starting with no knowledge -- most six year old kids lack the temperament to study and also in general have less ability to pick up concepts.
On the other hand, if it were a foreign language, the younger the better with some aspects of language being very hard to learn the older a human gets.
I note that the youngest Polgar became the best, and I think she not only started earliest but she obviously had the advantage of two older siblings who knew the game very well. It is also argued that the younger one starts, the slower the decline in old age. Some people became GM level starting in their teens or even 20s but the ones I have in mind did not play to late old age. (I wonder if Morphy, had he continued to play into middle and even old age would have been like Korchnoi, a top player as he approached 80 -- Morphy quit and passed away so young that he seems very much part of the mid 19th century but had he lived to his 90's, he could have potentially played with people who were 20th century world champions including certainly Lasker but also even Botvinnik -- imagine in the mid 1920s Morphy in his 80s meeting a young Botvinnik as Korchnoi met a young Caruana.)
