Team formation for soccer is based upon age groups.  US Youth Soccer and PA Youth Soccer set age groups to help ensure fair and competitive play in youth soccer.

All players on a team need to be under (abbreviated by "U") the required age on Jan 1st of that year. 

The age grouping does not match completely with the local public school age calendars.  It occasionally happens that a child may not be able to play with the other children in their school grade.