Boulogne:

“To achieve marked progress in sport requires specialization and each sport should be judged independently.

  In those sports where the participant has to master complicated technical skills, specialization should start early. But with others where the movements and disciplines are less complicated, this specialization can and should wait.

 Specialization in soccer needs to start early, as the players need to become familiar with and master the technical skills and the need to understand the numerous tactical nuances in the game. This is not the case in athletics.”