A child under the age of 6 must wear a USCG-approved Type I, II or III personal flotation device while onboard a vessel under 26 feet in length while the vessel is underway. "Underway" is defined as anytime except when the vessel is anchored, moored, made fast to the shore or aground.
FEDERAL LIFE JACKET RULE
The United States Coast Guard's Life Jacket Rule for Children went into effect December 23, 2002. In support of the ongoing efforts of the states and Coast Guard to improve boating safety, the Coast Guard is requiring that all children under 13 years of age wear Coast Guard approved life jackets, while aboard recreational vessels underway, except when the children are below decks or in an enclosed cabin