There are many ways to define biological/organismal aging, here I collected together 6. They are presented in historical order to show how different topics showed up in them and thinking on aging gained more depth. You should not expect strict definitions in the logical sense with necessary and sufficient conditions but attempts to highlight the most important characteristics and differentiate them. I’ve added 2 more columns: scope/complexity and actionability. Scope/complexity tries to assess the proposed definitions according to how comprehensive they are in capturing this most complex biological process of aging. I’ve focused on things explicitly mentioned in these attempts but with a little interpretation more elements can be dug out and added making definitions more similar and converging. Actionability refers to the usability of the definition to offer strategies to counteract these processes, please note that actionability is not something you would expect from a biological definition. The last 2 definitions were made by offering a divide and conquer approach, thereby enriching the biological attempts with a technological/engineering flavour.
Sources of definitions:
Medawar PB (1952) An Unsolved Problem of Biology: An Inaugural Lecture Delivered at University College, London, 6 December, 1951 (H.K. Lewis & Co, London).
Comfort, A. (1979). The Biology of Senescence. New York: Elsevier.
João Pedro de Magalhães: senescence.info
Carlos López-Otín, Maria Blasco, Linda Partridge, Manuel Serrano, Guido Kroemer (2013): Hallmarks of Aging
Aubrey de Grey: in Juvenescence: Investing in the age of longevity
Attila Csordas: wait a sec, that’s me. See my earlier post. I’ve learned the most from the Hallmarks of Aging paper and from Aubrey. Personally I love the scope of João Pedro’s definition and recommend that whole page highly. But I realised that the complexity and diversity of the process is not highlighted enough hence I try to offer a ‘Plurality view’ that immediately lends itself to divide and conquer algorithms. And no, I don’t think my position logically denies organismal aging being one biological and observable process, but it tries to focus on it’s being a bundle of processes, many of them independent from each other, as far as current science can tell. This might change later as deeper interconnectedness can be unveiled, but by now I’d like to focus on the whole package and stress it’s heterogenous underpinnings. One more factor of heterogeneity, that has not been included in earlier definitions explicitly, is the different rates at which separate components age biologically.