I am not necassarily in disagrrement that a infant should wear a PFD, but it is not required by law in Maryland so your statement is incorrect.
"Effective April 1, 2010, Maryland law states that all children under the age of 13 must WEAR a United States Coast Guard approved Personal Flotation Device (life jacket - Type I, II, III or V) while underway on a recreational vessel under 21 feet in length on Maryland waters. Recreational vessel includes motorboats, sailboats, canoes, kayaks, rowboats, and any other device capable of being used for transportation on the water, when the vessel is being used for other than commercial purposes. The life jacket must be the proper size for the child and must be in good and serviceable condition.
This requirement does not apply when:
A vessel is moored or anchored.
A child is below deck or in an enclosed cabin.
Also a child under the age of 4 must wear a PFD which features additional safety precautions, as appropriate for an infant, toddler, or young child, so as to:
hold the child securely within the PFD, including a strap that is secured between the child's legs to fasten together the front and back of the PFD;
maintain the buoyancy of the child, including an inflatable headrest or high collar; or
ensure the ready accessibility of the child from the vessel, including a web handle.
This does not apply to a vessel that is moored or anchored or a child who is below deck or in an enclosed cabin. "
Maryland Department of Natural Resources - Boating
Of course conditions/sea state, where you are sailing, type of boat, etc should all be considered when choosing where, when, what a child/infant should be in a PFD or Harness