[grondilu]

> If all of a hydra's cells are replaced every four weeks, I would say that the hydra has a life span of about four weeks. Ship of Theseus and all that.

Most of our cells in our bodies are renewed in a few months or something. Yet people don't assimilate this to death.

Now, you may object that neurons don't follow this rule, so our subjective identity is not concerned by this. To this I will say that when people are concerned about ageing, they are indeed concerned about what happens to their brain (that is, neuro-degenerative diseases), but certainly not only that. The state of all the other tissues is at stake : the heart, the bones, the skin and so on. Yet all of these are renewed on a regular basis. That renewal is not perfect though, and it's that imperfection they blame on ageing.