It is vital for player development that players at the younger ages (u7 through u10) experience ALL positions on the field. Coaches should use small-sided games (4v4/5v5) to help players develop technically and build a foundation for tactical awareness on the field.
A breakdown of implementing positions:
u7/u8 - players play all positions/zones on the field.
u9/u10 - players play all positions including goalkeeper.
u11/u12 - players look to narrow their position down to a side of the
field (right/left/central) or a zone (forward/midfield/defense) based on
their game effectiveness (Technical/Tactical Skills) and physical development.
u13 - players get to experience playing their side or zone on a larger
scale (11v11). Effectiveness on larger fields will be associated with Technical/Tactical Skills, athleticism, or the development of teammates.
U14+ players are set in a position that they/their coach deems their most favored/effective. They have the potential of being utilized in other areas of the field in playing roles.
Just remember, the one theme that remains constant is that players change as they get older - Some players get better, some players get worse, some players play the same way (but everyone else changes around them). Its the coaches job to help place ALL players in the best positions for the team and the individual.
Quan PhanDirector of Youth Development