I think the first step is to agree that anti-aging research would be a very significant (and possibly the best) way to be beneficent. It would do a lot of good, and be one of the best things that we could do. I think Bostrom is pretty successful in establishing this.
The second step, which Bostrom simply takes for granted, is the utilitarian idea that there is a "screaming moral imperative" to do a lot of good if we are able. I guess non-consequentialists won't be convinced by this.
I take it you're objecting to the second step, rather than the first?
To which I responded as follows:
Actually, I am inclined to object to the first as well. It's certainly true that anti-aging research could be one form of beneficence, and that the hoped-for result, if obtainable, would be good. But I don't think Bostrom does a very good job at all in showing that it would be one of the best things that we could do. First, because the way in which it would be a good thing is actually fairly common -- if we don't allow that aging, as such, is an evil (i.e., an evil in itself, independently of the question of how healthily we age), eliminating aging would actually be a fairly ordinary sort of medical hope, with lots and lots of competitors even on the purely medical side (curing cancer, eliminating the danger of flu viruses, wiping out malaria, improving the world blood supply and distribution, etc., etc.). When we add non-medical factors, as we would have to do if we were to compare it to things like eliminating war or improving global education or putting an end to world hunger, it starts to look very ordinary indeed. And, further, when we are considering how beneficent something is, Mike B. [one of the commenters] is quite right (and Bostrom quite wrong) that we can't just ignore potential negative consequences; purely for comparative purposes we need to know how extensive and how likely they are, because that affects how much good the action actually involves.
I think the only reason it sounds like it would be one of the best things we could do is that it would be a very big and showy thing to do, that would require a lot of changes to society. But we really shouldn't let this influence our evaluations. I confess, too, that I'm inclined to think the very vastness of the proposal suspicious; it's exactly the sort of flashy project First-Worlders propose and play up in order to make themselves feel good despite the fact that they are ignoring other problems that are more immediately serious, more fixable, or more someone-else's-concern. Sure, it would be good, but it also sounds suspiciously like a project of the "Ooh, we can't take the trouble to send a few mosquito nets to Africa to save lives, but that's OK because we're going to put a man on Mars" type.
Plus, I'm not really convinced that most people who are enthusiastic about it are really enthusiastic about it because they can eliminated age-related deaths in (say) Botswana rather than in their own immediate circle. I agree for some people it really would be a work of beneficence; I suspect that for most people, however, it's just a selfish fantasy distracting from the serious works of beneficence.