The three-age system is the periodization of human prehistory into three consecutive time periods, named for their respective predominant tool-making technologies:
The Stone Age
The Bronze Age
The Iron Age
The system is most apt in describing the progression of European and Mediterranean societies, although it has been used to describe other histories as well. The system has been criticized for being too technologically determinist.
Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element therein, after oxygen and silicon.
it is just like saying "but no copper age", "no gold age" if you search the wikipedia "three-age system" you will notice this type of arguments from different part of the world. Remember also the "ice age".