"HMS Victory is the only remaining 18th Century warship anywhere in the world and is the oldest serving Royal Navy ship in commission - she remains a fully commissioned ship with her own complement of officers and crew and is the flagship of the Second Sea Lord, Commander in Chief Naval Home Command.

HMS Victory was ordered in 1758, the same year in which Horatio Nelson was born at Burnham Thorpe in Norfolk. The keel was laid at Chatham Dockyard in 1759 with Victory being completed six years later. Her construction took 27 miles of rigging, 4 acres of sails and required 2,000 mature trees. She cost £63,176 to build. HMS Victory saw almost constant service from 1778 to 1812."
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""Old Ironsides" is the oldest commissioned warship afloat. USS Constitution is one of six ships ordered by President George Washington to protect America's growing maritime interests in the 1790s. Constitution soon earned widespread renown for her ability to punish French privateers in the Caribbean and thwart the Barbary pirates in the Mediterranean. The ship's greatest glory came during the War of 1812 when she defeated four British frigates.

Manned by an active duty U.S. Navy crew, Constitution is open year round for free public tours in the Charlestown Navy Yard. In the summer of 1997, Constitution sailed under her own canvas for the first time in 116 years"

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"Continental gunboat Philadelphia is the only surviving gunboat built and manned by American Forces during the Revolutionary War. Part of a hastily constructed fleet, she is one of 15 small craft with which Benedict Arnold fought 29 British vessels off Valcour Island in Lake Champlain in October 1776. "


OK, Victory IS the oldest, but not the only 18th century warship.