In the Tariff act of 1930, the U.S. Customs defined an antique as an object that was made before 1830 when mass production became commonplace. This act determined if a piece was duty free. In 1966, the standard of 100 years old was adopted as the defining characteristic to determine if an object was an antique and its import would be duty free. Prior to this act, importers would claim all sorts of items as antiques to avoid the tax.
In 1993 the Customs Modernization Act, Tile VI of the North American Free Trade Agreement Implementation Act, added to the definition and defined an antique (for purposes of import taxation) as a piece less than 50% restored or refinished so long as the “essential character” of the piece remains unchanged.