Aging is poorly defined as anything other than an outcome, as is pointed out in today's open access paper. Aging is the rise in mortality risk due to intrinsic causes over time, a definition that is hard to argue with, but that provides next to no insight. There are more complicated and detailed definitions, but near all are all quite similar in being a catalog of symptoms rather than a catalog of processes. The SENS view of aging, on the other hand, is in fact a list of causative processes, but is by no means widely agreed upon, nor expected to remain unchanged by later data.
Definitions of aging that are in effect a taxonomy, in the sense that we put these humans into the "old" bucket because they exhibit measurable symptoms A through Z, are really not helpful at all when it comes to the development of therapies to treat aging. One needs to know what to target. What processes must be interrupted? What damage must be repaired? This is the entire point of the SENS view of aging: that we need this description of aging as a set of processes, rather than as a set of symptoms, in order to even start building effective rejuvenation therapies.
Sadly, validating a consensus definition of aging is only going to be achieved via the production of working rejuvenation therapies. If one repairs a form of damage and that actually works to produce rejuvenation, then that is supportive of definitions of aging that include that specific form of damage. It seems likely to me that there will be active debate over the causes of aging all the way through the process of producing a first generation package of good-enough rejuvenation therapies that adds decades to healthy life spans. That active debate will take place alongside a great deal of wasted research and development effort, as people attempt to validate incorrect approaches.
The SENS view of aging is very important, not just because its conclusions as to areas of study and forms of therapy seem likely to be the best way forward to practical human rejuvenation, but also because, philosophically, everyone else should stop building taxonomies of aging and start thinking about causes and processes of aging. It will be a messy process all the way from here to the goal of physical immortality and medical control of aging, but it seems that there could be a lot less wasted effort along the way than is presently the case. Incidentally, the paper noted here starts off well, and is an interesting read, but then falls into the pit of suggesting that aging is an intrinsic property of life, and thus not amenable to effective treatment.
Several recent publications, including the one entitled "What if there is no such thing as aging?", have brought out an astonishing trend: the mere existence of biological aging is being questioned. The title reiterates the famous proposition "There is no such thing as aging and cancer is not related to it", and the paper suggests that the same is relevant to the other age-associated conditions: "...we are not studying a single biological phenomenon, but an assortment of loosely related processes that we find convenient to lump together"; ultimately, "... the concept of aging does not reflect any underlying biological reality".
Being intentionally provocative, the paper does however reflect the trend, which is developing, paradoxically, in parallel with the increasing prevalence of aged people in the world and with the recently emerged discipline called "geroscience". According to the geroscience agenda, the best way to combat the most prevalent age-associated conditions, such as atherosclerosis, cancer, type II diabetes, and neurodegeneration, is to target their common risk factor rather than each of the conditions specifically. The common risk factor is aging.
Apart of that the very practice of piling up of newly invented scientific disciplines and respective terms is questionable, a problem with the geroscience agenda relates to the feasibility of evaluating the benefits of its implementation. The benefits of targeting of a disease may be evaluated, based on its commonly accepted definition (diagnostic criteria), as a decrease in the incidence of cases recognized according to this definition. Can we decrease the incidence of aging otherwise than by making people dead before they get old? Any answer to this question depends on the criteria used to distinguish (i) aging from all the rest that may occur to living bodies and (ii) living bodies from all other kinds of bodies, in other words, on the definition of (biological) aging. The lack of consensus on the definition of aging is long recognized and has recently been highlighted by asking several basic questions about aging to recognized authorities in aging research. Answers to each of the questions differed up to antipodal extremes.
Based on several scores of definitions found in the most highly cited (that is the most representative and influential) papers on aging, the following features commonly used to define aging have been distinguished: (1) structural damage, (2) functional decline, (3) depletion of a reserve required to compensate for the decline, (4) typical phenotypic changes or their cause, and (5) increasing probability of death. Noteworthy, these characteristics are not really five definitions of aging, but rather five defining features of aging. This the above inventory to the kind of definitions that in philosophy of science is known as "nominal" and is opposed to "real"
By its nominal definition, water is a colorless and odorless liquid having defined specific gravity, viscosity etc. By its real definition, water is a compound comprised of two hydrogen atoms and one oxygen atom connected in a certain order. Noteworthy, the real definition is senseless for people ignorant of atoms. Likewise, the nominal definition of aging as a set of observable features should be supplemented, if not replaced, with its real definition. The latter is suggested here to imply that aging is the product of chemical interactions between the rapidly turning-over free metabolites and the slowly turning-over metabolites incorporated in macromolecules involved in metabolic control.
The phenomenon defined in this way emerged concomitantly with metabolic pathways controlled by enzymes coded for by information-storing macromolecules and is inevitable wherever such conditions coincide. Aging research, thus, is concerned with the elucidation of the pathways and mechanisms that link aging defined as above to its hallmarks and manifestations, including those comprised by its nominal definitions. Esoteric as it may seem, defining aging is important for deciding whether aging is what should be declared as the target of interventions aimed at increasing human life and health spans.