Based on the past couple of years, I think that the following science is worth watching in 2006:
1) Mitochondrial research
The tools for manipulating mitochondria - our cellular power stations - and associated technology demonstrations have become much more impressive of late. Given that mitochondrial dysfunction is strongly linked to a number of diseases as well as age-related degeneration, advances in this area offer a great deal of potential.
2) Commercialization of stem cell therapies
The gate is already open, and the first stem cell medicine companies are already offering limited services. I'm expecting to see a range of first generation stem cell therapies, mostly using adult stem cells obtained from the patient, become the subject of commercial efforts outside the US during 2006.
3) Calorie restriction biochemistry
2005 was a year of progress, learning and unexpected twists for research into the mechanisms of calorie restriction and other links between metabolism and longevity. Look for more of the same to come.
Given that a number of venture-funded groups will enter their second year of investigating calorie restriction biochemistry and genetics in 2006, we should soon be hearing about initial attempts to apply this new knowledge to the treatment of diseases. That said, I imagine it will look very much like old school drug development; trying to find compounds that will flip the right cellular triggers (for some subset of the population, with a few side-effects as possible). Our ability to look and understand outpaces our ability to manipulate - and greatly outpaces the stifling regulatory regimes tied to the status quo and last century's medical development process.
4) Gene therapies and immunotherapies
Both gene therapies and immune therapies seem to be moving quickly from "promising, but no results" towards "working therapy in hand." I would be very surprised if 2006 passes by without important developments and further attempts to cure age-related conditions using these methods.
5) Activism for real anti-aging research
If real, meaningful anti-aging research isn't funded, then the end result - longer, healthier lives - won't come to pass in time to help us. If the regulatory process makes it impossible to approve or market therapies to address the aging process, then research won't be funded. We have a long way to go in terms of steering the research community and funding organizations towards a full-press assault on the problem of degenerative aging. While many helpful or related fields of medicine show progress, we have to move beyond incidental life extension if we want to see a cure for aging within our lifetime.
Fortunately, the efforts of organizations like the Methuselah Foundation are making an impact - we must keep up the good work and continue our support for greater funding, freedom of research, and change in the way scientific community regards longevity research.