In writing about the scary existentialist method, it occured to me that there is one other major limit to longevity. As you get older, it requires more and more complexity in algorithm to operate your mind. It has been scientifically proven that older people aren't thinking any slower (unless they have a specific illness), they're sorting greater amounts of information. Logically, then, if you don't upgrade your cognitive powers, you will reach a point where your CNS simply cannot produce an algorithm complex enough to correlate all its component data.
Solutions: Forget stuff older than a certain date? That seems a bad solution. Some method of augmenting the mind - obviously, high Cog characters have an advantage here, but how does a low-Cog character get upgraded without simply wiping away who they once were? (The Scary Existentialist Method isn't really a solution to that - it's built on the notion that personality and personal identity are seperable concepts.)