There is a law but it ends after they are 7 years old. Seems kind of young to me.
Taken from here - My linkhttp://www.boat-ed.com/mo/handbook/pfds.htm
Personal Flotation Devices (PFDs)
All vessels 16 feet in length or longer must carry one wearable U.S. Coast Guard—approved PFD (life jacket) for each person on board or being towed. Vessels less than 16 feet in length must carry one wearable or one throwable USCG—approved PFD for each person on board or being towed. Under federal law, however, a wearable PFD is required for each person on board regardless of vessel length.
Children under 7 years of age must wear a USCG—approved PFD at all times while on board any vessel, unless the child is confined in a totally enclosed area of the vessel such as the cabin area of a houseboat or day-cruiser.
One USCG—approved Type IV personal flotation device must be on board vessels 16 feet or longer and readily accessible, in addition to the above requirements.
Each person riding on a PWC must wear a USCG—approved Type I, II, III, or V PFD.
Besides being USCG—approved, all PFDs must be:
In good and serviceable condition.
Readily accessible, which means you are able to put the PFD on quickly in an emergency. PFDs may not be stowed in closed or locked compartments.
Of the proper size for the intended wearer. Sizing for PFDs is based on body weight and chest size.