Is it ever explained how kids in the academy are so athletic at their age?

Like, a fully trained ninja, sure. But is it ever explained why kids in the academy can do the basic athletic maneuvers (Jumping from branch to branch, running faster than avg, etc) while other people their age can't?

Is it related to chakra? Are they just physically conditioned since the time they enter (6-8 I think?)? Is it just suspension of disbelief bc the setting says so?