Ageing Biology: Mechanisms, Hallmarks, and Longevity Science

Introduction to Ageing Biology

Biological ageing is the progressive decline in physiological function with time, increasing vulnerability to disease and death. While ageing is universal, its rate varies enormously between species (bdelloid rotifers vs. mayflies vs. bowhead whales) and between individuals. Ageing is not programmed per se but rather reflects the accumulation of molecular and cellular damage over time, coupled with evolutionary neglect of late-life maintenance after reproductive success declines. Understanding ageing mechanisms offers the prospect of compressing morbidity—reducing the period of disability at end of life—or extending healthy lifespan (healthspan).

The biology of ageing has been transformed by the discovery of genetic interventions dramatically extending lifespan in model organisms. Single-gene mutations can double lifespan in nematodes (daf-2, the insulin/IGF-1 receptor) and extend mouse lifespan by 30-40%. Evolutionary conservation of these longevity pathways across species suggests they can be translated. Hallmarks of ageing—Lopez-Otin's framework identifying molecular mechanisms including genomic instability, telomere attrition, epigenetic alterations, proteostasis loss, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication—provide a unifying organising framework.

Molecular Mechanisms of Ageing

Genomic Instability and DNA Damage

Accumulation of DNA damage throughout life contributes to ageing. Sources include reactive oxygen species from mitochondrial respiration, replication errors, telomere erosion, and exogenous mutagens. DNA repair capacity declines with age; the balance between damage accumulation and repair capacity shifts unfavourably. Werner syndrome, Cockayne syndrome, and Hutchinson-Gilford progeria—progeroid syndromes with mutations in DNA repair, nuclear lamina, or RecQ helicase genes—cause premature ageing phenotypes, demonstrating that DNA integrity maintenance is essential for normal ageing rate. Extremely long-lived species (bowhead whales, naked mole rats) have enhanced DNA repair capacity.

Epigenetic Ageing Clocks

DNA methylation patterns change predictably with age, forming the basis of epigenetic clocks—accurate predictors of biological age from methylation at hundreds of CpG sites. Horvath's 2013 clock predicts age within a few years from saliva, blood, or tissue cells. Acceleration of epigenetic age relative to chronological age predicts mortality, cancer risk, and cognitive decline better than chronological age alone, suggesting it reflects true biological ageing rate. Parabiosis experiments connecting young and old mice partially restored young methylation patterns in old mice. Yamanaka factor partial reprogramming (transient expression of Oct4, Sox2, Klf4) in aged mice reversed epigenetic age while maintaining cell identity.

Longevity Pathways

Insulin/IGF-1 Signalling and TOR

Reduced insulin/IGF-1 signalling extends lifespan in C. elegans (daf-2 mutations), Drosophila, and mice by activating FOXO transcription factors upregulating stress resistance and repair genes. Nutritional restriction (caloric restriction, intermittent fasting) consistently extends lifespan across organisms by reducing TOR (target of rapamycin) activity and activating AMPK. Rapamycin, the TOR inhibitor, extended mouse lifespan by 9-14% even when started late in life (equivalent to a 60-year-old human starting treatment)—the strongest pharmacological lifespan extension in mammals. These pathways connect nutrient sensing to longevity regulation.

Sirtuins and NAD+ Biology

Sirtuins are NAD+-dependent deacetylases (SIRT1-7 in mammals) regulating metabolism, stress response, and ageing. NAD+ levels decline with age (~50% by midlife); NAD+ precursors (nicotinamide mononucleotide, NMN; nicotinamide riboside, NR) raise NAD+ levels and improve metabolic function and physical performance in aged mice. SIRT1 activates autophagy, mitochondrial biogenesis (PGC1-alpha), and DNA repair. Resveratrol activates SIRT1 mimicking caloric restriction effects in rodents; human trials showed more limited benefits. NMN supplementation trials in humans showed NAD+ restoration and modest metabolic improvements—larger trials are ongoing.

Cellular Senescence

Cellular senescence is a stable cell cycle arrest triggered by telomere erosion, DNA damage, oncogene activation, or oxidative stress. Senescent cells resist apoptosis yet secrete a pro-inflammatory senescence-associated secretory phenotype (SASP)—cytokines, proteases, and growth factors that promote tissue inflammation, stem cell dysfunction, and paradoxically can promote cancer in neighbouring cells. Senescent cells accumulate in tissues with age. Senolytic drugs (ABT-263/navitoclax targeting BCL-2/BCL-xL; dasatinib+quercetin) selectively kill senescent cells; first-in-human trials in IPF patients showed reduced senescent cell burden, circulating SASP markers, and trends toward functional improvement. Senescence is both an anti-cancer mechanism and a driver of ageing.

Examples and Applications

Example 1: Rapamycin and Lifespan Extension

The ITP (Intervention Testing Programme) found rapamycin extended median lifespan 9-14% in genetically heterogeneous mice even starting at 600 days (equivalent to 60-year-old humans). The mechanism involves mTORC1 inhibition reducing protein synthesis, increasing autophagy, and activating stress responses. Intermittent rapamycin reduces immunosuppressive side effects. Clinical trials of intermittent low-dose rapamycin in elderly humans (PEARL trial) assess immune function improvement without infection vulnerability. Dogs (a more translatable model) are enrolled in the Dog Ageing Project testing rapamycin for healthspan extension.

Example 2: Naked Mole Rat Longevity

The naked mole rat lives 37+ years—10 times longer than mice of similar size—without exhibiting traditional ageing hallmarks. It maintains proteostasis superiority (highly active proteasome, superior unfolded protein response), shows sustained cancer resistance (contact inhibition through high-molecular-weight hyaluronan, unusual p16/p27 expression), maintains telomere length without teleomere shortening, and exhibits minimal senescence accumulation. Understanding its unique biology—concerted longevity mechanisms not found in short-lived rodents—reveals potential interventions for human healthspan extension. Genome sequencing identified molecular differences in damage response and antioxidant systems.

Example 3: Supercentenarians and Genetic Longevity

Individuals living 110+ years (supercentenarians) and their first-degree relatives show enrichment of genetic variants in DNA repair genes, immune function genes, and lipid metabolism. APOE2 allele halves Alzheimer's risk versus APOE4 which triples it—explaining part of longevity heritability. FOXO3A variants associated with longevity across multiple populations regulate stress response downstream of insulin/IGF-1 signalling. Genome-wide studies of exceptional longevity are limited by small sample sizes; integrating epigenetic age data with genomics improves power to identify longevity determinants beyond random survival.

Example 4: Parabiosis and Young Blood Factors

Heterochronic parabiosis—surgically conjoining old and young mice to share circulation—improved multiple ageing phenotypes in old mice: muscle regeneration, neurogenesis, cardiac function, and cognitive performance. Young plasma contains factors promoting rejuvenation; old plasma contains inhibitory factors. GDF11 (initially proposed as a youthful muscle rejuvenation factor) remains controversial. Anti-ageing factors in young blood include TIMP2, beta2-microglobulin absent, and GDF11. StartingUp companies are conducting trials of young plasma administration; regulatory caution about unproven plasma treatments prompted FDA warnings, illustrating the science-commercialisation tensions in longevity research.

Example 5: Caloric Restriction and Fasting

30% caloric restriction extends lifespan 30-40% in rodents and improves nearly all biomarkers of metabolic health. CALERIE trial—25% caloric restriction in healthy humans for 2 years—improved metabolic health markers, reduced inflammation, and slowed thymic ageing. Time-restricted eating (16:8 fasting pattern) improves insulin sensitivity, reduces inflammation, and may slow ageing through mTOR inhibition during fasting periods. The CALERIE trial established proof-of-principle that caloric restriction retards human biological ageing; whether lifespan is extended in humans with healthier baseline habits than rodents is untested.

Example 6: Metformin and Ageing (TAME Trial)

The TAME (Targeting Ageing with Metformin) trial is the first FDA-accepted clinical trial explicitly targeting ageing rather than a specific age-related disease. Over 3000 adults aged 65-79 will receive metformin for 6 years with composite endpoint of major cardiovascular events, cancer, dementia, disability, and death. Metformin activates AMPK, inhibits complex I, reduces hepatic glucose production, and reduces mTOR activity. Epidemiological data from diabetics show metformin users outlive type 2 diabetes-free controls. If TAME succeeds, metformin could become the first drug approved as an anti-ageing intervention, transforming regulatory frameworks for longevity medicine.

Example 7: Partial Reprogramming

Yamanaka factors (Oct4, Sox2, Klf4, cMyc) overexpressed briefly—partial reprogramming not proceeding to pluripotency—reversed epigenetic ageing marks in mouse retinal ganglion cells restoring vision in aged/glaucomatous mice and in systemic administration improving multiple tissue functions. Human embryonic fibroblasts and endothelial cells partially reprogrammed showed reduced epigenetic age and improved function. Concerns about oncogenesis from c-Myc and ongoing reprogramming prompted development of alternative factor combinations (SKO—Sox2, Klf4, Oct4). Clinical translation requires demonstrating safety in primates before human trials can proceed—the most advanced longevity intervention at the frontier of development.

Example 8: Ageing and Alzheimer's Disease

Alzheimer's disease predominantly afflicts the elderly—prevalence doubles every 5 years over 65, suggesting ageing itself is a driver beyond genetic risk. Ageing-related changes in brain clearance (reduced glymphatic flow during sleep, impaired autophagy, neuroinflammation) contribute to amyloid and tau accumulation. Restoring sleep quality (glymphatic clearance occurs preferentially during deep sleep) is a modifiable factor. TREM2 variants increasing microglial amyloid clearing reduce AD risk; enhancing microglial phagocytic function is a therapeutic strategy. The convergence of ageing biology and AD pathology suggests that anti-ageing interventions targeting senescence, neuroinflammation, and proteostasis may reduce AD risk as a secondary benefit.

Try it live

Everything above runs in your browser — open Cellular Ageing & Senescence Simulator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.