David Appell has an amazing and provocative interview with biologist Aubrey de Grey, a "biogerentologist" who thinks a 5,000-year human life span is within reach:
TR: 5,000 years? That seems pretty outlandish.De Grey: I get that reaction a lot - my estimate gives people the conceptual bends. But if you go through the logic step by step, you'll see that it's virtually inevitable that we will attain that sort of life expectancy in that sort of timeframe, just so long as two things work out. First, we will have to develop first-generation rejuvenation therapies by 2050 or sooner. Secondly, the value we put on life will have to rise as our anticipated lifespan increase, which has happened in the past. I'm not a sociologist, but I think this second development is very likely, given the first.
So, what remains for me to explain is why the development of first-generation rejuvenation therapies - which I'll define as ones that can be applied to people in their 60s and increase their lifespan by at least 30 years - is enough to give people who are 25 or younger at the time those therapies arrive a lifespan not limited by aging in any way. The answer can be given in one word: bootstrapping. Thirty years is an absolute eternity in science, so it is nigh on inconceivable that the second-generation therapies (which, let's say, give another 30 years on top of what the first-generation ones give) will be so long in coming that the beneficiaries of first-generation therapies will miss out on them. And of course the same applies to subsequent-generation therapies ad infinitum.