It is from Outliers by Malcolm Gladwell. In this book he suggests that this happens because at a young age these kids born in January have an age advantage over their peers.
During adolescence physical development can be a substantial advantage and one born in January compared to another born in August will be physically different in terms of development.
Therefore, they are better from the start due to physical advantage. This results in more coaching and more attention, therefore they are encouraged to progress further in baseball.
