This article looks past the immediate challenges of aging and early medical biotechnologies aimed at extending human longevity, and into the future of merged molecular manufacturing and biotechnology, when it will become possible to replace our biology with far more robust and long-lasting machinery:
If we're talking far-future, non-biological approaches to life-extension will win out over biological approaches, due mainly to their comparative advantages (e.g. ease of repair and modification). [I] think that the distinction between non-biological and biological systems (especially if Drexlerian nanotech - that is, using mechanosynthesis - is implemented with any ubiquity) will increasingly dissolve. If a system exhibits the structural, functional and operational modalities of a biological cell, tissue, organ or organism, yet consists of wholly inorganic materials, is it not closer to a biological system than to what we would typically consider a non-biological system? Either the distinction between the two will eventually dissolve, or we will use the term "biological" to designate systems exhibiting the structural, functional, and/or operational modalities of biological systems.
I make a distinction between life-extension therapies and indefinite-longevity therapies, and I'd like to elaborate more on this distinction here. Life-extension therapies extend longevity, but for various reasons fail to make it necessarily indefinite or unlimited. Often this is because such therapies aren't comprehensive - a given therapy solves one contributing factor of aging, but not all of them. Others, like SENS (which I'm in no way discounting), fix the major causes of damage, but use a different methodology for each respective source of damage or aging; the drawback of this approach is that if previously overshadowed causes of aging now begin to make a non-negligible impact on aging, in the absence of the more predominant causes, then we have no methodology to combat it. Because each strategy is tied intimately to the cause it seeks to ameliorate, the techniques often cannot be applied to the new source of molecular damage.
Indefinite or unlimited longevity therapies, on the other hand, use one comprehensive approach to mitigate all sources of aging. One example is Drexlerian nanotech (and to a shared but somewhat lesser extent Robert Freitas's nanomedicine - only because it has specifically-tailored strategies not dependent on the feasibility of Drexlerian molecular assembly or "mechanosynthesis", in addition to the more comprehensive ones). This approach fixes not the source of the damage but the damage itself, iteratively, and can thus be used to combat any source of molecular damage using the same tools, technologies and techniques. With such therapies we wouldn't need to come up with a second wave of strategies to combat those sources of aging that might crop up in the future, and which remained unnoticed until such a time only because their impact couldn't be seen (or allowed to take effect) while the first wave of sources was still predominant.
I'm not totally convinced that this last point is the case; I think it's more that a designed replacement for tissue can be made to have far fewer and more comprehensibly understood forms of aging (which can be repaired on an ongoing basis). But there will still be the unknowns, pushed into an ever-smaller corner, and ever less important. Yet by the time it is possible to build artificial tissue and cell replacements in this way, will we not have come to understand biology so well that the unknowns in biological aging are already equally small?