WordNet records no synonym and no antonym for golden age. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a time period when some activity or skill was at its peak
“it was the golden age of cinema”
any period (sometimes imaginary) of great peace and prosperity and happiness
(classical mythology) the first and best age of the world, a time of ideal happiness, prosperity, and innocence; by extension, any flourishing and outstanding period
Synonyms, antonyms and the broader and narrower relations for golden age come from Princeton University’s WordNet. Full sources and licences.