Why do we age? The question is deeper than it sounds. From an evolutionary perspective, aging is not an obvious feature for natural selection to produce — organisms that don't deteriorate would, all else equal, leave more descendants. Yet almost all multicellular life shows progressive deterioration culminating in death, with characteristic species-specific lifespans ranging from days (mayflies) to centuries (Greenland sharks, bristlecone pines). The modern theory of aging — assembled across the twentieth century by Peter Medawar, George Williams, and Tom Kirkwood — explains why selection allows aging to exist; the contemporary biology of aging focuses on how it happens at the molecular and cellular level, and increasingly whether and how it can be slowed. The 2013 paper The Hallmarks of Aging (López-Otín et al.) compressed decades of work into a taxonomy of nine (later twelve) interlocking processes and made aging biology one of the most active research frontiers in medicine.
The evolutionary theory of aging rests on three connected ideas: Medawar's mutation accumulation (1952) — selection's strength weakens with age (most reproduction has occurred by mid-life), so deleterious mutations whose effects manifest only late are less strongly purged; Williams's antagonistic pleiotropy (1957) — alleles that help reproduction in youth but harm survival in old age can be positively selected because youth-fitness gains outweigh old-age losses; and Kirkwood's disposable-soma theory (1977) — selection apportions limited resources between reproduction and somatic maintenance to maximize lifetime reproductive success, sacrificing some maintenance for reproductive output. The molecular hallmarks of aging (López-Otín et al. 2013, updated 2023) identify twelve interlocking processes: genomic instability from accumulating DNA damage; telomere attrition as chromosome ends shorten with each division; epigenetic alterations in DNA-methylation patterns and histone marks; loss of proteostasis and disabled macroautophagy in protein quality control; deregulated nutrient sensing (altered insulin/IGF-1, mTOR, AMPK, sirtuin signaling — the most modifiable hallmark); mitochondrial dysfunction; cellular senescence with the senescence-associated secretory phenotype releasing inflammatory factors; stem-cell exhaustion; inflammaging; chronic inflammation; and dysbiosis in the gut microbiome. These hallmarks are interconnected — telomere shortening triggers senescence, senescent cells produce inflammation, inflammation drives further damage, mitochondrial decline impairs proteostasis — and the major lifespan-extending interventions in model organisms all hit these pathways. Caloric restriction (~30% reduction without malnutrition) extends lifespan in yeast, worms, flies, mice, and partially in primates through nutrient-sensing pathways; rapamycin (an mTOR inhibitor) extends lifespan in mice even when started in middle age; metformin (acting on AMPK) is being tested in humans (the TAME trial); senolytics (dasatinib + quercetin, navitoclax, fisetin) selectively kill senescent cells and extend healthspan in mice; and Yamanaka-factor reprogramming (Oct4, Sox2, Klf4, c-Myc partially expressed) can reverse some aging hallmarks in mice, with partial-reprogramming protocols an active research frontier.
Aging is increasingly being treated as a tractable medical problem rather than an inevitable background condition. The NIH National Institute on Aging now funds substantial geroscience research aimed at modifying the aging process itself rather than treating individual age-related diseases, and major commercial players (Altos Labs with $3B+ in 2022 funding focused on cellular reprogramming, Calico under Alphabet, Unity Biotechnology on senolytics, Retro Biosciences on reprogramming) have entered the field. FDA-track interventions include rapamycin and metformin (off-label use is widespread, large RCTs ongoing), senolytics (multiple Phase 2 trials), and NAD+ precursors (NMN, NR — popular as supplements, evidence modest), with epigenetic clocks (Horvath, GrimAge, PhenoAge, DunedinPACE) increasingly used as surrogate endpoints. The honest current assessment is that no human intervention has yet been demonstrated to extend maximum lifespan, several plausibly extend healthspan, and whether aging will become a meaningfully modifiable condition in this century is one of biology's largest open practical questions.