Law 7 - Duration of the Game/Official Timekeeping/Tight Schedule
U-19 and U-18 play 45-minute halves; U-16 play 40-minute halves; U-13 and U-14 play 35-minute halves; U-12 and younger play 30-minute halves.
The referee is the official timekeeper, and he/she may add time to compensate for obvious time-wasting, prolonged injury, or time lost by excessive substitution.
The referee and the coaches must recognize that every field is tightly scheduled and that all games must start on time. Youth referees are typically assigned to more than one game, sometimes at different locations. However, if a delayed start time is beyond the control of both teams, all efforts should be made to play a complete game, without disrupting the subsequent games at the same site.