Subsequent authors were always changing up the content of their predecessors to keep things fresh. (Simply repeating Hesiod wouldn't get an audience excited. Contradicting previous authors or adding new twists to the old stories sells books, or scrolls, as the case may be.)
On the other hand, Ovid may have simply been attracted to the idea of comparing ...
Hesiod describes the Five Ages in his poem Works and Days (lines 109 - 201) 1. Their order is:
Golden Age, ruled by Cronus,
Silver Age, when Zeus rule begins,
Bronze Age, an age of tough men that ends with the flood of Deucalion,
Heroic Age, when the Trojan War occurs,
Iron Age, the current age.
Hesiod does not mention a future age past his own, which ...
I believe this version is a bit more illuminating. Grey at birth seems to imply by the surrounding text to mean a belief they are mature before their time.
Thereafter, would that I were not among the men of the fifth generation, but either had died before or been born afterwards. For now truly is a race of iron, and men never rest from labour and sorrow ...