In different sports, a “career” begins at different ages: from three years in acrobatics, at a fairly mature age in auto and motor sports. In general, we can say this: the more difficult the sport is from a technical point of view, the later they get into it.
But it is important to understand that whenever and wherever a child goes, he should already be physically developed sufficiently. Both doctors and trainers - all as one they say that you need to start familiarizing with sports with swimming, and already from 2-2.5 years.
Firstly, it is very useful from a physiological point of view: swimming develops all muscle groups. Secondly, to be able to swim is a matter of life and death, both in a figurative sense (and you yourself try to survive among peers in the country, when you are the only one who can not swim in-about-that tree!), And in direct. For a child who knows how to swim, and have to worry much less.
Another good option for harmonious development since childhood is acrobatics. In China, for example, children are sent to the sports section in acrobatics as early as 3 years old, and after a couple of years a commission of trainers decides which sport the kid should go next, depending on his natural data.