Why are we sad - if we are - when a famous person dies at a ripe age?

I just read posts on another site lamenting the death of Chinua Achebe. I wonder what the significance of the death of a famous person is to those who mourn them. Obviously, it is one thing when a public figure is cut down in the middle of their productive life – like Janis, Jimi or JFK – but what gets triggered when a famous person dies at a reasonable age? I don’t think it generally has much of an effect on me. Just some Friday afternoon musings. Your thoughts?