I think the first Golden Age started in 1986 with the original Kong animation on the tour. What followed was Earthquake, ET, BTTF, Jurassic Park, and T23D. So late 80s - Late 90s was USH's first golden age in my eyes, and will always be my favorite period in USH's history for multiple reasons. I was introduced to the park as a toddler in 1996 and remember it all vividly, that's when USH has world class rides and attractions all based on their own popular films, and just in general it was all amazing during that time.
The second "Age" definitely started with the new Kong, then Transformers, and so on. I guess we can call the Early 2000s as the Dark Ages of USH.