The Simple Answer is the Right Answer
Straightforward thinking from the Singularity Institute Blog: "If a young child falls on the train tracks, it is good to save them, and if a 45-year-old suffers from a debilitating disease, it is good to cure them. If you have a logical turn of mind, you are bound to ask whether this is a special case of a general ethical principle which says 'Life is good, death is bad; health is good, sickness is bad.' If so - and here we enter into controversial territory - we can follow this general principle to a surprising new conclusion: If a 95-year-old is threatened by death from old age, it would be good to drag them from those train tracks, if possible. And if a 120-year-old is starting to feel slightly sickly, it would be good to restore them to full vigor, if possible. With current technology it is not possible. But if the technology became available in some future year - given sufficiently advanced medical nanotechnology, or such other contrivances as future minds may devise - would you judge it a good thing, to save that life, and stay that debility? ... If you believe professional bioethicists (people who get paid to explain ethical judgments) then the rule 'Life is good, death is bad; health is good, sickness is bad' holds only until some critical age, and then flips polarity. Why should it flip? Why not just keep on with life-is-good?"
Link: http://www.singinst.org/blog/2007/06/16/transhumanism-as-simplified-humanism/