There are a lot of factors. Happiness isn't a big factor, but it is a factor. Various policies and relgious beliefs can add up. My current Denmark game I've been in a permanent GA for over 100 turns with no great artists used for GA simply because hero worship (the founder belief that gives GA points and Faith when you conqure a city) has given me a trillion GA points.
Aztecs can get golden ages simply by killing city states fwiw.
Also the tier 1 autocracy policy that gives culture and GA points when your borders expand gives me enough points for a permanent GA on it's own with 20+ cities, God of Expanse, Angkor Wat and Denmark.