The topic in question is fuzzy/indeterminite. The transition to widespread acceptance from marginalization was not instantaneous. The work currently ongoing began sometime after Resveratrol's antiagapic effects were discovered.

I don't get that impression from my (limited) knowledge of the field. If Stipp's book is accurate then there have been many attempts in the last hundred years by serious scientists.

Also, in regards to the issue of premises, are you asserting that if scientists in 1900 or 1950 had put in current levels of attention into anti-aging that they would have succeeded at the level you estimate?

No. I have no such claim to make. Ok. Based on that and your other remarks then you have to be working off of not just your generic, timeless premises: 1) That humans are material organisms.
2) That organisms follow the laws of biology.
3) That there is no supernatural force dictating human events.
Etc.,

but also specific estimates for the current technological level and general knowledge level. So the question becomes then how confident are you in those estimates? Pedictions of sweeping change are usually wrong. Such changes do sometimes occur from technologies, but most of those are things like ubiquitous cell phones or the internet or GPS which are not heavily anticipated or are anticipated in only a rough fashion. The outside view suggests that predictions about major technologies should have their confidences reduced.

There's an unstated premise about how much medical/biochem knowledge we have now.

No, there really isn't. It's not about that. It's about topical focus. It's about old age itself being seen as a topic worthy of being disassembled and its mechanisms researched in order to find solutions to that effect.

This can't be all that matters if the same result would not have occurred in 1950 or 1900 (or even more to the point 1850). That means that at least one premise needs to be not just about topical focus but about the overarching technological level. To use a weak analogy, post 1945 or so it woud be fair to say that the only issue preventing a major power from getting to the moon was one of topical focus. But that's because one is taking into account the general tec level. I couldn't make that claim in 1900 or 1850. And in defending that difference, I have to be able to point to specific technologies that they had in 1945 that they didn't in 1900.

It seems to me that you are arguing in a way that is useful to convince someone who assigns a very low probability that lifespan extension will occur that they should assign a high probability. That's not a point I disagree with. The issue is your claim that:

I cannot conceive of a worldline resultant from the here-and-now that would not include the datapoint that within fifty years from today, antiagapics research had extended human lifespans by at least another fifty years.

This is in context a bit stronger than this because you seem to be claiming not that people born fifty years from now will have a lifespan extension of another fifty years but that this will apply to people who are already old. This is a difficult position since with the exceptions of prosthetics and tissue-cloning, most of the plausible methods of life extension (such as reservatrol) rely on starting the process when people are young. Even if we have some form of reservatrol-like compound in fifty years that slows aging down to a crawl, someone who is a hundred years old at that point will still have the cells of a centenarian (assuming minimal prior anti-aging treatments).

Moreover, one can easily conceive of circumstances where many of the proposed anti-aging systems just won't work. For example, humans have a much longer average life-span than most mammals. It could turn out that humans already do something to their cells that mimics most of the effects of reservatrol.

There's also a general evolutionary issue at work here: If a species has a maximal lifespan of around L, then if there are any ways to get more reproductive fitness at a younger age that create problems sometime around or after L. So, one should expect not just one thing to go wrong as a species ages, but lots of things to go wrong. In fact, this is close to what one sees. If this problem is severe enough, it is possible that there are diseases which will show up in the very elderly that we haven't even noticed yet because the population of such people is just too small.

To further this discussion, over at Prediction Book I've made a series of predictions about age extension related predictions. I'm curious how much and where your estimates disagree with mine. (I'm not including separate links to each, but the ones on that list are the reservatrol predictions and the sets about oldest living people).