This will be the last in my series of columns on aging and longevity research. I've spent so many pixels on it because I think the topic is about to take off in importance, both in terms of progress in research, and in terms of political debate. I've already written about the politics of research funding, and about the shaky link between death and dynamism. And last week I had an interview with researcher Aubrey de Grey, who thought that, among other things, my estimate of doubled lifespans this century was much too pessimistic.

Now I'd like to play clean-up, and address a few issues that have come up along the way. One has to do with quality of life. I once saw Jay Leno deriding the benefits of diet and exercise. What they don't tell you, he said, is that all that will get you is added years in your 80s, when what you really want are added years in your 20s.

That's not strictly true for diet and exercise, of course -- they tend to get you not only more years, but better years. But it's likely to be far less true with anti-aging treatments. Treated with anti-aging therapies, eighty-year-olds may not look like twenty-year-olds, but they won't turn into modern day versions of Swift's struldbrugs, either, who lived longer but only in increasingly decrepit condition.

The decrepitude of age, after all, exists as the result of, well, aging. Indeed, contemporary medicine, which keeps people alive longer but which can't address the aging process itself, is far more likely to produce such a state of affairs -- though today's elderly are not all decrepit by any means. (Zack Lynch thinks we'll need more treatments for age-related mental defects -- though to the extent that aging process can be stopped or reversed, we might wind up needing fewer.)

Whether longevity research will give people the equivalent of extra years in their twenties or not -- and I think it just might -- it's quite likely to dramatically extend healthy middle age. Certainly that will be the goal. As Alexander Capron writes in an essay in Coping With Methuselah: The Impact of Molecular Biology on Medicine and Society, a book recently published by the Brookings Institution, the ideal aging treatment would extend the prime-of-life middle years, while, if anything, shortening the period of late-term decline. That's what people want, that's what drug companies will try to develop, and that's what we'll likely see eventually. Drugs that produce struldbrugs won't be popular, and probably won't ever find much of a market.

Which in part answers the next question: How will we pay for retirement? The answer, it seems to me, is obvious: We won't.

Today's notion of "retirement age" is a fairly recent one. Otto von Bismarck is often credited with craftily setting the retirement age at 65 because most people wouldn't live that long -- though in fact, Bismarck set it at 70, and it wasn't lowered to 65 until later. But the justification for retirement has always been that by retirement age people were nearly used up, and deserved a bit of fun and then a comfortable and dignified decline until death. Get rid of the decline and death, and you've given up the justification for living -- as Social Security recipients, at least, do -- off other people's efforts on what amounts to a form of welfare . Logically, retirement should be put off until people are medically old, or perhaps just replaced with disability, and those who are able to work should do so, while those desirous of not working should save up as for a long vacation.

In this regard, increased longevity, with (at the very least) much higher retirement ages, could be the salvation of many nations' pension systems, which to varying degrees are facing an actuarial disaster already as the result of longer lifespans and lower retirement ages, coupled with lowered birthrates.

Indeed, although many people worry that longer lifespans will lead to overpopulation, the world is now facing what Phillip Longman, writing in Foreign Affairs, calls a "global baby bust." Longer lives and later retirements will help offset at least some of the consequences of falling birthrates -- and at least conceivably, people who expect to live longer will be more willing to take time out early to bear and raise children, without feeling that it's such a career sacrifice to do so.

Finally, there are religious objections to longer life. Those aren't likely to be universally shared, even among believers -- one emailer noted that he thought efforts to achieve longer lifespans were religiously-required, because "the longer a person lives, the more time he or she has to become a Christian." It's hard to argue with that. And certainly we read of enormously long lifespans in the Bible (Methuselah, for example) with no suggestion that there's anything wrong with them. As I noted earlier, though, some people are sure to object, because there are always people with religious objections to, well, anything. But others will disagree on moral grounds, and I rather doubt the religious objectors will carry the day.

At any rate, I plan to give this topic a rest for a while, lest it grow, er, old. But I do believe that the research is going to progress rapidly over the next few years, and that the public debate on the subject is going to take off as well. Stay tuned.