Final answer:
The xiphoid process is the most inferior portion of the sternum, which ossifies from cartilage starting in middle age.
Step-by-step explanation:
The xiphoid process is the inferior most aspect of the sternum. This small structure starts as cartilaginous tissue in early life but undergoes ossification, turning into bone starting during middle age. The sternum, a key part of the thoracic cage, is divided into three parts: the manubrium, the body, and the xiphoid process. Its role is significant as it anchors ribs through the costal cartilage and provides attachment for various muscles.