Various myths involve deities dying (such as Baldr and Osiris). However, in most of the myths that come to mind, another deity is responsible for the death. In Christian belief, Roman soldiers executed Jesus, but this is a relatively late example.
How old is the trope of a mortal (including demigods, heroes, etc, anyone who has some amount of human ancestry and is able to die of old age) defeating and killing a god? What is the earliest known instance of it?