Complex sentence: Born on a Sioux Indian reservation in Minnesota, Burnham learned the ways of Native Americans as a boy, and at 14 he was already supporting himself in California while also learning scouting from some of the last of the old cowboys and frontiersmen of the American Southwest.
Split into simpler sentences: Born on a Sioux Indian reservation in Minnesota, Burnham learned the ways of Native Americans as a boy. By the age of 14 he was supporting himself in California while also learning scouting from some of the last of the old cowboys and frontiersmen of the American Southwest.