I buy and sell old antiques, antiques are my business.Here are the signs by which an item can be classified as an antique:
Age (at least 50 years); at the same time, unique items between 15 and 50 years old (for example, things from the Soviet era) can be classified as vintage.
Rarity (that is, the rarity or uniqueness of a thing);
(not) serial;
Connection with the historical era or with historical events (that is, to what extent the item reflects the fashion trends of its era);
(Im)possibility of reproduction;
Artistic/historical/cultural value;
Material value.