This study, "A Phase II Study of Cabozantinib (XL l84) for Plexiform Neurofibromas in Subjects with Neurofibromatosis Type I in Children and Adults diagnosed with Neurofibromatosis Type 1 (NF1) and have a type of tumor called a plexiform neurofibroma (PN). Neurofibromas are tumors that develop from the cells and tissues that cover the nerves. Plexiform neurofibromas can be disfiguring, painful, and life-threatening. These types of tumors typically do not respond well to most treatment approaches such as chemotherapy, radiation, and surgery because of their slow growth and location near vital structures of the body such as nerves, blood vessels, and the airway. The primary objective is to determine the response rate of NFI patients with plexiform neurofibromas treated with Cabozantinib therapy using MRI scans. The objective response rate to cabozantinib is defined as ≥ 20% reduction in tumor volume at the end of 12 cycles.