Virtually every athlete on teams like Real Madrid or the Chicago Bears came up through a youth system or showed promise while playing in high school or college, with many of them already well versed in their sport of choice by the time their age hits double figures.
It's the same in most top-level individual sports — think Tiger Woods age 3 up against Bob Hope on TV, or the Williams sisters in a tennis academy before they had even finished elementary school.
The notion of a person walking off the street into a major tournament asking to be part of the draw, or knocking on the door of a major football team with no experience in the game but getting a tryout that same day is laughable.