The stone age was when people hit things with stones.
The bronze age was when people hit things with bronze.
The iron age was when people hit things with (you guessed it!) iron!
However odd it sounds it is true. But here is the technically correct explanation.
The change from the stone age to the bronze age was a gradual change. The bronze age was when the most used material in manufacturing was bronze. The bronze could be from either smelting copper and tin (that were extracted from ores) to make bronze or just trading with other countries.
Answer 2: The Bronze Age happened because the tools made with Bronze were superior to tools made with the materials that were used earlier (Stone and/or Copper).
In most societies tool-making technology evolved from: Stone Tools => Copper Tools => Bronze Tools. Bronze is superior to Stone and Copper because:
it’s more durable, harder than copper and less brittle than stone
Bronze is alloy made up of two metals either copper & arsenic or copper & tin. These metals are relatively easy to smelt (smelting is the extraction of a metal from its naturally occurring ore)
It’s relatively easy to forge (forging is heating the metal and pounding it into the shape of the tool you want)
It’s castable (casting is melting the metal and pouring it into a mold to create the tool you want)
It makes better edged and pointed tools (Axes, hatchets, knives swords) than copper or stone. Many of these are weapons and better weapons means societies that use Bronze had an advantage on the battlefield.