Final answer:
The sacrum is formed by the fusion of five sacral vertebrae which begin to fuse around the age of 20 and are typically fused into a single bone by middle age.
Step-by-step explanation:
The sacrum is a triangular-shaped bone in the vertebral column that is formed by the fusion of five sacral vertebrae. These vertebrae are initially separate during early development, but they begin to fuse around the age of 20 and typically complete fusion by middle age. In an adult, the sacrum acts as a critical component for weight bearing at the base of the spine and provides a point of attachment for various muscles and ligaments.