2. Introduction

Aging is the dominant risk factor for most forms of functional decline and most chronic diseases and, in modern human populations, it is the major underlying determinant of mortality. As our civilization brought many early-life hazards under better control, the burden of late-life disorders became increasingly visible, shifting attention from acute causes of death to a broad spectrum of age-associated decline. This is why aging now sits simultaneously at the center of biology, medicine, public health, and social policy. Yet aging is unlike most biomedical problems: it is not a single disease, cannot be reduced to individual genes or pathways, and is not even a term on which the field fully agrees. At the same time, the possibility of modifying aging is no longer merely speculative. Lifespan varies greatly across species and cell types, many interventions can extend lifespan in model organisms, and biomarker-based approaches increasingly allow biological aging to be tracked in humans.

Despite this progress, the field remains conceptually fragmented. Different investigators define aging as accumulation of damage, decline of function, loss of homeostasis, loss of regulation and coordination, maladaptive remodeling, increase in biological age, continuation of development, increasing mortality risk, or some combination of these. There is a broad disagreement on the most basic issues, including what aging is, what causes it, when it begins, whether rejuvenation is possible, and whether consensus is even necessary (1). Such plurality of views can be productive, but it also has costs: when investigators use the same word for different processes, comparisons across models, biomarkers, and interventions become blurred, and interpretation of findings is challenging. One study may claim to slow aging because it lowers an aging clock output, another because it delays pathology, a third because it slows functional decline, and a fourth because it changes survival. These are overlapping but not identical claims.

A large body of literature has attempted to explain aging through particular causal lenses. Evolutionary theories emphasized the declining force of natural selection with age and the trade-offs that limit maintenance, as in mutation accumulation, antagonistic pleiotropy, and disposable soma. Other frameworks stressed specific classes of deleterious change or response, including molecular damage, regulatory drift, hyperfunction, cellular senescence, and more recently the hallmarks of aging. These traditions have each been valuable, but they often answer different questions: why maintenance is limited, what forms of deleterious change accumulate, or which phenotypes best represent aging. The result is a field rich in mechanisms and paradigms, yet poor in a shared conceptual language that would allow these ideas to be compared directly, assessed on the same map and ultimately experimentally validated or disproven.

This article presents the author’s views on central questions in aging biology through brief, stand-alone topics. Its four sections cover fundamentals, measurement, practical interventions, and longer-term strategies. Additional topics appear in the SI Appendix.