The proper adjective for Antarctica is Antarctic, as in Antarctic Circle.
Antarctica is a proper noun. It is the name of a continent.
The proper adjective for Portuguese is "Portuguese."
The proper adjective for southwest is "southwestern."
The proper adjective for Haiti is Haitian.
Vietnamese is the proper adjective for Vietnam.
The proper adjective for Siam is Siamese.
The proper adjective for Georgia is Georgian.
The proper adjective for Mayan is "Mayan."
The proper adjective for George is Georgian.
Freudian is the proper adjective for Freud.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.