I agree that with certain collectables, like cars, they seem to have different criteria. maybe other things as well.
I do not presume to know everything about the subject but, the general consensus is that most items fall under the criteria I posted above, for instance:
Merriam-Webster:
Definition of
antique
1: a relic or object of ancient times
2a: a work of art, piece of furniture, or decorative object made at an earlier period and according to various customs laws at least 100 years ago
b: a manufactured product (such as an automobile) from an earlier period
Dictionary.com
noun
any work of art, piece of furniture, decorative object, or the like, created or produced in a former period, or, according to U.S. customs laws, 100 years before date of purchase.
the antique style, usually Greek or Roman, especially in art.
Cambridge Dictionary:
made in an earlier period and considered to have value because of being beautiful, rare, old, or of high quality.
Importing Art into the US
In the majority of situations, original works of art are exempt from import fees or duties.
Chapter 97 of the HTSUS defines original artwork as art that the original artist created, exclusively by hand and without any mechanical process. Therefore, mass-produced reproductions are not eligible for the same duty-free exemption.
Importing Antiques Into the US
Antiques are exempt from import duties. To qualify, antiques must be at least 100 years old and have documentation included that confirms the age. Imported antiques found to not be at least 100 years old can be subject to an import duty between 5.2 and 25 percent depending on where the antique came from.
All imported items are subject to the following Tariff code guidelines:
Harmonized Tariff codes for importing antiques is as follows:
9706.00.00 = antique item more than 100 years old
9705.00.00 = collectible item of historical and ethnographic interest
9703.00.0000 = Original sculptures and statuary, in any material
