Have you asked even one? Have you asked any experts at all? This is what is so puzzling to me about this line of questioning--is the question actually prompted by acquaintance with the subject, or is it of a piece with doubting any other scientific claim? What if the projection is correct, even though you have trouble believing it? I have very little trouble believing the projection, having a mother born in the 1930s who has passed her eighty-first birthday, and having an aunt (her older sister) born in the 1920s who has passed her ninety-fourth birthday. Plenty of people from much earlier birth cohorts are already living into extended old age, so what is so daring about a prediction that anyone who meets established long-term patterns of life expectancy will live into an era with more progress in disease prevention and treatment?
(In short, why have you cited no evidence whatever on this point?)
(1) the fact that the mortality rates at elderly ages has leveled off for many decades in developing countries and (2) that almost all the increases in life expectancy comes from reducing infant mortality and the spread of infectious diseases.
That dual statement suggests you have not looked carefully at the link in my original footnote 4
http://www.nature.com/scientificamerican/journal/v307/n3/box...
(Nature's repost of a chart originally available at the Scientific American website, but a dead link there now), in which it is shown that life expectancy has been increasing steadily at ALL ages, including old ages. You really need to read up more on this, or it's hard for anyone to discuss these issues with you here. You are very careful to speak factually on other issues here on Hacker News, so this is taking me a bit by surprise. Do you know of any experts in this domain at all? Whose writings do you recommend?
Finding one expert who presents the conditional claim of another expert as unconditional, and (you) then asserting this claim as accepted fact is highly misleading. This is especially true when that unconditional claim is almost certain to be disputed by the majority of experts. Giving your citations reduces the friction necessary for people to identify how you are being misleading, but it's not a blanket pass to mislead.
Furthermore, it won't always be the case that there will be a nerd like me willing to dive into your citations to show why they don't support your claim. In this case, the veneer of authority that comes with listing many citations can easily overwhelm the benefit they provide, leading to more misinformation than if your speculative claim had been uncited and therefore ignored.
> How would one prove this claim wrong?
You would ask a few of experts if they think it's plausible that life expediencies would follow the same trajectory in developed countries this century as they have the past century. I conjecture that almost all those experts would reply "no", probabaly citing (1) the fact that the mortality rates at elderly ages has leveled off for many decades in developing countries and (2) that almost all the increases in life expectancy comes from reducing infant mortality and the spread of infectious diseases.
Indeed, the soft ceiling on human longevity has not budged much since Psalms 90:10 was written about 3 millennia ago:
> The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.