In The Giver, at what age are you chosen for volunteer hours? What age does it end?

In Lois Lowry's novel The Giver, children begin volunteer hours after their Ceremony of Eight. At this Ceremony, the children begin their metaphorical initiation into adulthood. Their comfort objects are taken away from them, and they are given jackets with pockets which symbolize their newfound responsibility for keeping track of their own possessions. They begin their volunteer hours by selecting an area of service where they would like to give their time. Despite the pretense of free will that is offered up in this choice, these hours serve as a "test run" for their future career within the Community. The children are carefully surveyed by the Elders, who will decide whether or not the selected area of service would be a good fit for the child's actual job. 

This job, or "assignment," is given at the Ceremony of Twelve. It is at this point that the children have completed all their volunteer hours, ending their volunteer work, and start an actual job.