In my opinion, it all depends on the individual characteristics of the child. A huge number of examples of "early maturing" suggests that you can start with 3-4 years. More on this can be found on the website http://chess555.narod.ru/volsh1.htm as in other sports(Yes!) "children develop a sense of responsibility and fighting qualities, without which it is impossible to achieve high results. Still, in my opinion, the best age to start training is 5-6 years.
Early practice not a guarantee of future success - it is enough to recall distant socialist times, when almost every kindergarten Lviv hung a banner type "to first grade - with first category". Later, however, the vast majority occurred stagnation at the level of master. So I would caution parents from hasty conclusions - even as a new player in 10 years to make significant progress. 6-th world champion Botvinnik learned to play at age 11, but by the age of 14 he became a master,and at 16 was the finalist of the championship of the USSR, which in those days was the equivalent of the GM class.
Moreover, even in adulthood, it is possible to progress my students raised the rating from 1700 to 2200 in 38-45 years! Of course, this is not enough for the professional game, but it is quite another level. So, if you have certain abilities, desires, and with some effort you can achieve a very tangible success. Chess is not a mystery. And I'll prove it to you!:)