Suppose you believe, as I do, that major breakthroughs in medical technology (from cloning, stem cells, etc.) are likely to occur within the next 20 or 30 years, allowing dramatic extensions of the human lifespan. How should this affect your behavior? Should you take less care of yourself, or more? The simple answer is that, since you think the new technologies will probably save you, you can risk a greater amount of unhealthy activity. But this answer is flawed, for at least two reasons.
First, there’s a question as to exactly when the advances will occur. You need to make sure you live long enough, and stay healthy enough, to take advantage of them when they finally arrive. You want to be the fellow riding the wave, not the guy who misses the swell and gets left behind while everyone else rides it in. (I hope Tom is not cringing at my amateur use of a surfing metaphor.)
Second, healthy living and medical technology might be complements, not substitutes. For instance, if they start growing cloned organs in labs for transplantation, you have to be healthy enough to survive the transplantation process. Or the new technologies might excel at life-extension but lag when it comes to quality-of-life. If you fail to maintain your body in ways that allow you to enjoy it, you could find yourself echoing the words of John Cougar Mellencamp: “Oh yeah, life goes on / Long after the thrill of livin’ is gone...”
A related thought: Some people can’t understand why I’d want to live forever (or for a really long time). I think those people must not enjoy living that much. But I recognize the possibility that I’ll get tired of life one day. And so I say to the naysayers, “It’s not necessarily about living forever; it’s about having the option.”