 Because his society has decided that he's too old, so does it just dispose of him? As though his life no longer had any value or meaning. In my opinion, this is exactly the hidden meaning of cliches like, the old must make room for the young. Common wisdom would have it that there is some vaguely defined age when you've had your time and your existence is no longer as meaningful or important as that of somebody younger. It sounds harsh, unmean and cruel, and it is! And that's precisely why I think we hide it behind idealized and romanticized figures of speech like walking into the sunset or passing on the torch.