Lazer's right, about 5 or 6 for most kids, as soon as they can have fun. For best results in any sport, as early as possible. A.J. Liebling on boxing:
It is true there exist certain generalized conditions today, like full employment and a late school-leaving age, that militate against the development of first-rate professional boxers. (They militate also against the development of first-rate acrobats, fiddlers, and chefs de cuisine.) “Drummers and boxers, to acquire excellence, must begin young,” the great Egan wrote in 1820. “There is a peculiar nimbleness of the wrist and exercise of the shoulder required, that is only obtained from growth and practice.” Protracted exposure to education conflicts with this acquisition, but if a boy has a true vocation he can do much in his spare time. Tony Canzoneri, a very fine featherweight and lightweight of the thirties, told me once, for example, that he never had on a boxing glove until he was eight years old. “But of course I had done some street fighting,” he said to explain how he had overcome his late start.