Some day soon, perhaps in forty years, there will be no one...

Some day soon, perhaps in forty years, there will be no one alive who has ever known me. That's when I will be truly dead - when I exist in no one's memory. I thought a lot about how someone very old is the last living individual to have known some person or cluster of people. When that person dies, the whole cluster dies,too, vanishes from the living memory. I wonder who that person will be for me. Whose death will make me truly dead?