What is an "Antique?"
In the Tariff Act of 1930, the U.S. Customs defined an antique as object that was made before 1830 when mass production became commonplace. In 1966, the standard of 100 years old was adopted as the defining characteristic to determine if an object was an antique and it's import would be duty-free. Before this Act, importers often claimed all sorts of objects as antiques to avoid the tax.
On December 8, 1993, Title VI of the North American Free Trade Agreement Implementation Act (Pub. L. 103-182, 107 Stat. 2057), also known as the Customs Modernization or 'Mod' Act, became effective. These provisions amended many sections of the Tariff Act of 1930 and related laws. One key change to the Act concerns restoration.
"Provided they retain their original character, the heading includes antique articles that have been repaired or restored. For example, the heading includes antique furniture incorporating parts of modern manufacture. However, if the essential character is changed, or more than 50% of the item has been repaired or restored, the item is no longer considered an antique and is subject to duty."
Are Antiques Valuable?
Being old, even 100 years or more, doesn't automatically mean furniture or other wooden objects are valuable. What does determine value is its rarity, quality, condition, and provenance.
In one episode of Antiques Roadshow, one of the participants had a small painted box from Pennsylvania. Even though it didn't seem exceptional in appearance or construction, it was very valuable because of it's rarity and very good original condition. In another episode, a woman brought in a small round scallop table that was extremely valuable because of it's excellent condition, known craftmanship, and rarity. Most old furniture simply doesn't meet these criteria.