easywhat am-i

This thing all things devours: Birds, beasts, trees, flowers; Gnaws iron, bites steel; Grinds hard stones to meal; Slays king, ruins town, And beats high mountain down.
Common Wrong Answers
“Death”
While death does consume all living things, it does not literally affect non-living entities like iron or stones, which the riddle suggests.
“Nature”
Nature does influence and change things, but it does not specifically devour or grind down like time does.
“Age”
Age is a concept related to time, but it does not encapsulate the all-encompassing, destructive power attributed to time in the riddle.