A team can only play effectively, if it is organized. The youth clubs and academies of leading soccer nations have clearly defined the field sizes, numbers of players, team formations, and game rules that must be suitable for the given age of children. The soccer society suggests that it is the age of 12 when children should start playing in a full-sized field and this is the age when they can move into the league system from a tournament system.
But to get this far is a long way.