Complex sentence: Rockwell's family moved to New Rochelle, New York, when Norman was twenty-one years old and they shared a studio with the cartoonist Clyde Forsythe, who worked for "The Saturday Evening Post".
Split into simpler sentences: Rockwell's family moved to New Rochelle, New York, when Norman was 21 years old. They shared a studio with the cartoonist Clyde Forsythe, who worked for "The Saturday Evening Post".