Compare the following two scenarios:
A) Everyone lives comfortably for 80 years.
B) Everyone lives comfortably for 120 years, but with a correspondingly lower birth rate.
We stipulate that both scenarios have the same number of people living at any one time, and the same aggregate welfare. This means that in case B, fewer people will get to live, but each of those who do will get to live for longer (an extra 40 years to be exact). I think that scenario B is plausibly better for each of the individuals who gets to live in it. But which scenario is better overall, from an impersonal point of view?
In simple utilitarian terms, there is nothing to distinguish them. Given the scenario as described, I'm inclined to think that's the right answer, too. But we might obtain more interesting results if we allow for some complications.
For instance, the "irreplaceable" nature of persons with future interests might lead us to prefer scenario B. It involves fewer deaths. Perhaps it is easier to realize greater value within a single unified life, than it is to obtain the same amount of value by spreading experiences across a range of people. (Here the value of a life might be seen in accordance with the Gestalt principle, that the whole is more than the sum of its parts.)
Or we might appeal to efficiency considerations, noting that society invests a great deal in raising children, so type B is to be preferred on the basis that it yields a better ratio of productive adult years for each year of childhood 'investment'. (This assumes that the extra 40 years are of sufficiently good health. Utilitarian considerations would seem to count most strongly against further "investment" in the elderly, for what returns do you get?) Kelly raised a similar point in pre-talk discussion.
Relatedly, it might be thought that the resulting higher median age in B would lead to a "wiser" society.
Conversely, we might prefer a type-A society with more "fresh blood" coming through with new ideas and creativity. Apparently most top scientists do their best work when younger (in their twenties?). And it has been said that progress occurs not by changing people's minds, but simply by having all the stubborn old-timers die out! So perhaps the long-lived society would risk stagnation, and we should prefer a fluctuating population with shorter life-cycles?
Another possible advantage of scenario A is that more people get to live (and we have stipulated that these are lives worth living). If we hold that 80 years is sufficient for a very good life, and the extra 40 years are mere luxury, then perhaps it would be more "generous" to share the years around a bit, and give more others a chance to live as well. This is a difficult suggestion to make clear sense of, since you presumably cannot have obligations to non-existent beings. But perhaps a related idea could be expressed in impersonal terms, by suggesting that a world with more happy lives is, in that respect, a better world.
Overall, I remain undecided. I think life-extension is probably a good thing, because it is good for the individuals that receive it, and it won't necessarily prevent any other goods from being realized (unlike in the thought experiment in this post, where we have assumed fewer new people would get to live). Nevertheless, it probably ought to be a lower priority than I previously suggested; it is, after all, a form of investment in the elderly, and -- callous though this may sound -- society's resources are probably better spent nurturing the young. But I'm certainly open to hearing any arguments to the contrary...?