Complex sentence: Keller began her modeling career when she was 15 years-old and has modeled for Revlon, GAP, Parasuco, L'Oréal, and for Sports Illustrated in 2002 and 2003.
Split into simpler sentences: Keller began her modeling career when she was 15 years-old. She was a model for Sports Illustrated in 2002 and 2003.