https://footballdna.co.uk/features/formation-analysis9-a-side/
In this tactical analysis feature the coach explores the progression from 7 a side into 9 v 9. The format of 9v9 is one in which we start seeing a more adult version of the game. The bigger pitches, off side rules and increase number of players on the pitch are introduced at a crucial age in child development.
At 11 and 12 years of age we will start to see the increase growth and maturation of some children, which can result in a significant physical gap between players. Therefore, often those that have matured earlier will be able to cope better with this transition, but may also benefit from being introduced into 11v11 earlier. When considering the 9v9 format, look at your players and think about what they need to help support their development.
Please follow the link to this page as its one of the best and in-depth tactical explanations I have found...