There is no consensus in aging research on how to define, measure, or explain aging, with fundamental disagreements over its causes, onset, and reversibility.

Examples of the diverse mechanistic definitions of aging.

“We do not know what causes aging. I think the trigger is endogenous but not necessarily programmed.” (G. Ferbeyre)

“Aging is mainly accumulation of macromolecular damage and failure of adaptation to a continuous changing environment. It is also an epiphenomenon of the outcome of the protective conditions we humans have achieved thanks to civilization over time and to this end somehow it doesn't comply fully with typical Darwinian Biology.” (E.S. Gonos)

“As an evolutionary biologist, I see 'aging' as the decrease in the age-specific contribution to fitness. […] At a mechanistic level, aging (or I should say 'senescence') corresponds to any deterioration of cellular/physiological traits that will ultimately impact fitness.” (J.F. Lemaître)

“Mechanistically, aging is a disruption of the homeostasis established between cellular processes. Age-related-diseases are the external symptoms of this disruption. […] Aging is the result of [an] imperfect optimization to maintain a balance between evolutionary constraints, cellular dynamical equilibrium and environmental constraints. […] Aging is not a programmed process but a consequence of this search for an equilibrium.” (Q. Vanhaelen)

“Time-dependent degradation of interactions or their (quasi)adaptative changes; sluggish and/or ineffective responses to challenges; when more than one such response is biologically relevant, aging would manifest as preference for a response different than one preferred at youth. The latter response may still be effective, but at higher cost for an aging organism (e.g. increased innate, inflammatory responses to pathogens in the old).” (J.M. Witkowski)

“Aging is the progressive decline of function with increasing chronological age due to internal factors that are not dependent on environment, which leads to an increased probability of death. Aging is caused by a genetically programmed switch that downregulates cellular pathways involved in homeostasis, stress response, repair etc. that evolved to limit competition for resources between offspring and parent.” (J. Van Raamsdonk)

“I believe that some mechanism that clears damage from the germline (which is immortal) i

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