Hamartia (noun) (literary)
The fatal flaw that leads to the downfall of a character.
Has its origins in late 18th century: Greek, ‘fault, failure, guilt’; the term was used in Aristotle’s Poetics with reference to ancient Greek tragedy.
“It took until the last chapter but the protagonist’s hamartia was revealed.”
The last stage of life; old age.
Senectitude comes from the Medieval Latin noun senectitūdō meaning “old age,” which in turn comes from Classical Latin senectūs, a derivative of the noun senex meaning “old man.” Senectitude entered English in the late 1700s, more precisely, in 1796 in Jonathan Swift’s Gulliver’s Travels