In order to complete the Bronze Age in Civilization, you must first have a city with a population of at least 500 citizens. Once you have this, you must then produce 1000 bronze coins. These can be produced by either mining for copper and tin, or by trading with other civilizations that have already reached the Bronze Age. Finally, you must research the technology Bronze Working. This will allow you to upgrade your units to use bronze weapons and armor, which are much stronger than their stone counterparts.