Complex sentence: He lived with his grandparents until he was 6 or 7, during which he was placed in one of the many boarding schools ran by the U.S. Bureau of Indian Affairs designed to "civilize" the Native Americans after their forced settling on reservations.
Split into simpler sentences: He lived with his grandparents until he was 6 or 7, after which he was placed in one of the many boarding schools run by the U.S. Bureau of Indian Affairs. These schools were designed to "civilize" the Native Americans after their forced settling on reservations.