Final answer:
Children are required to wear a PFD on a vessel under 21 feet until they reach the age of 13.
Step-by-step explanation:
Children are required to wear a Personal Flotation Device (PFD) while moving on a vessel under 21 feet until they reach the age of 13. According to boating regulations, children under 13 years old must wear a Coast Guard-approved PFD when on a moving vessel. This requirement is in place to ensure the safety of children in case of an accident or emergency on the water.