The "Golden Age of Mankind" refers to a time, before the Rifts, before the "Dark Age", when there were no such things as D-Bees, Mutants, Juicers, Crazies, Cyberknights, Vampires nor Magic.
In this time, science, technology and medicine began to combine to create cybernetics, bionics and human augmentation. This new development in human augmentation excited all the nations, and everybody began pouring resources into this new field. As the nations worked on these, trade of information and technology increased, making nations wealthy and peace reigned as each focused on themselves.
Once they had achieved great bounds in human augmentation, they began looking at what other nations were doing, and they noticed that not all nations had achieved equally. While everyone had succeeded, some had overshadowed others with what was created.
This led to discontent among the nations, then discontent led to suspicion. After all, what were these super-men good for, other than war? The fear and suspicion led to slowing of trade between nations, weakening the nation's infrastructure. This was seen as pre-emptive actions, as a weak nation is easier to conquor.
After some skirmishes, smaller nations joined the larger ones, creating more definite lines of power. Once these super-nations were formed, the governments started developing machines of war, creating; Glitter Boys, Juicers, M&M's, Borgs, Bots and Crazies.