I was wondering about usage of singularity and plurality of the word age. Why do we make it plural for the word age when we say the medieval ages or the dark ages? Or why is it the iron age and not the iron ages?
I suspect it is a matter of convention. The Middle Ages can, in fact, be broken out into separate periods - the Early Middle Ages, the High Middle Ages, and the Late Middle Ages - but even these, as you can see, are expressed in the plural. Furthermore, the Iron Age also has periods - Early, Middle and Late - but is spoken of in the singular. From this, I conclude (perhaps erroneously) that it is a matter of conventional usage rather than logic. As such, the phrases have to learned off pat rather than by applying a rule.
Perhaps - and this is pure conjecture on my part - it because stone, bronze, and iron are a single material, whereas Dark Ages and Middle Ages simply refer to (not well defined) periods in history. We call them Dark because we don't know much about them and we think they were barbaric. We call them Middle because they come between antiquity and the present. It appears that we now live in the Atomic Age (reference to nuclear power) or the Space Age.