Professional sports leagues are thought by many to be natural monopolies. This makes sense because fans of a sport want to have one overall champion in a sports. Who's the best? Since there is one champion, the most efficient number of leagues will be one.
How does this work with youth sports? In my town, the Mankato Area Youth Baseball Association (MAYBA) is the only amateur summer baseball league in town. They make their money by selling spots on teams to parents of young players. They field "in house" teams which play each other within the city and they field travel teams under the moniker of "The Royals" which play games against teams from other cities. In both types, there are teams for the various age groups, and in Royals there can be multiple teams within an age group, such as 15AAA, 15AA, 15A, and 15/14A. These are teams for 15-year olds and the more A's means a better overall team quality. 15/14 means the team is a mixture of 15 and 14 year olds. The AAA's play other AAA's, the AA's play other AA's etc.
In my town MAYBA is the only youth baseball organization,. but that doesn't make youth baseball organizations natural monopolies in general. To be natural, there needs to be global economies of scale. Is this the case with youth sports organizations? I'll just leave this post open ended.