Complex sentence: Perez began modeling at the tender age of 12 but it wasn't until she moved to New York at age 18 that she began doing high fashion jobs; debuting in New York Fashion Week for Betsey Johnson in September 2003.
Split into simpler sentences: Perez began modeling at the age of 12 but it was not until she moved to New York at age 18 that she began working in high fashion. She debute in New York Fashion Week for Betsey Johnson in September 2003.