As old as the hills

The phrase derives from the Bible, Job 15:7. It was alluded to in Miles Coverdale’s Bible, 1535:

Art thou the first man, that euer was borne? Or, wast thou made before the hylles?

The phrase in its current form didn’t gain use until the 18th century. The first example I can find of that form of words is in Francis Hutchinson’s A defence of the antient historians, 1734:

As vales are as old as the hills, so loughs and rivers must be as old as they.

It is quite possible that Hutchinson was making a literal reference to hills and not using the expression in its figurative form. A figurative usage does come not long after however, in The Edinburgh Magazine, 1787:

If an unlucky gamester brought on his papyrus a combination of letters already known, every body abused him saying “That has been already said” – “That is as old as the hills” – “all the world knows that”.

See also ‘As old as Methuselah‘.

See other ‘as x as y similes‘.