Seriously. Real increases in lifespan (I'm thinking about Louis Gridley Wu celebrating his 200 birthday) would have to involve (at least occassionally) revitalizing stem cells to renew the body at rates that would seem normal in a very young person. This would have to include nerve cells. Presumably, this would lead to a reversal of specialization. That implies memory loss ("memory" in all neurological senses: how to write software, but also how to speak English, how to control defecation, how to control urination. Really renewing one's central nervous system would rewind a person back to the unspecialized nervous system they were born with.)
Why would you think that? Information is stored in the configuration of neurons, not in the neurons themselves. Neurons are constantly being replaced, and you don't wake up one morning not knowing how to bike because your "bicycle neuron" died.
Are they ? As far as I know, neurons last more or less from before birth to death. There is neurogenesis in the hippocampus but it doesn't replace existing neurons, and the new cells themselves last until death.
