The Golden Age was a period in Cybertronian history under Sentinel Zeta Prime following the Quintesson invasion and preceding the Caste system. Basically, the Golden Age was a period in Cybertronian history on Cybertron when the world was rich in energon and prosperous and the inhabitants were at peace with one another. This was a time before the political corruption and war that left Cybertron in ruins.
When the Quintessons left Cybertron, they left behind spacecrafts and the designs for Space Bridges. Using these, the Cybertronians spread to the stars and colonized countless worlds, cyberforming them if necessary. Cybertron itself was booming with people and economy. Their science advanced greatly with discoveries such as Synthetic Energon. However, one ship encountered a rust plague, which spread through the Space Bridges killing millions. The Golden Age ended when Sentinel destroyed the Space Bridges to save Cybertron and imposed the Caste system. However, more privileged members of the Caste society felt it end when the Great War began.