Are those arguing for immortality assuming that aside from physical decline we improve monotonically with age? That's not at all clear to me. For example, the general consensus among linguists is that the ability to acquire native fluency in a language is lost after a certain age. Could there be other less obvious deficits in neuroplasticity that people striving for immortality would need to address? Sticking with the same example, how do we know it doesn't confer some evolutionary advantage to repurpose the language acquisition firmware (whatever that may be) to more age appropriate ends later? Oddly enough, a couple of star trek episodes I'm too lazy to look up got me thinking about all this. In one of them, captain Picard gets a chance to relive a regrettable incident from his youth and ends up ruining his life, and in the other, captain Janeway goes back in time to help her younger self and finds out they both have something to learn from each other. Relatedly, someone asked William Shatner what he wished he knew when he was younger and he said a better question would be about what he's glad he didn't know. Disclaimer: I had an unexpected sudden cancer scare last week so maybe you should discount my comment as a rationalization.