Why do we get old and die?

Genesis 3:1-5 recounts a story, an event that took place in the Garden of Eden. Gen3:1 Now the serpent was the most cautious of all the wild animals of the field that Jehovah God had made. So it said to the woman: “Did God really say that you must not eat from every tree of the garden?” 2 At this the woman said to the serpent: “We may eat of the fruit of the trees of the garden. 3 But God has said about the fruit of the tree that is in the middle of the garden: ‘You must not eat from it, no, you must not touch it; otherwise you will die.’” 4 At this the serpent said to the woman: “You certainly will not die. 5 For God knows that in the very day you eat from it, your eyes will be opened and you will be like God, knowing good and bad.” There was never a definition or explanation of what death was during this accounting of this event, so it must be concluded that both Adam and Eve knew what death was. Jehovah must have explained it to them at some earlier time. At Genesis 3:17-19 Jehovah further explains to Adam the sentence of death. It reads, 17 "And to Adam he said: “Because you listened to your wife’s voice and ate from the tree concerning which I gave you this command, ‘You must not eat from it,’ cursed is the ground on your account. In pain you will eat its produce all the days of your life. 18 It will grow thorns and thistles for you, and you must eat the vegetation of the field. 19 In the sweat of your face you will eat bread until you return to the ground, for out of it you were taken. For dust you are and to dust you will return.”