Ageing

• 1. Introduction

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  Of the roughly 150,000 people who die each day across the globe, about two-thirds die from age-related causes. That is around 100,000 deaths a day traceable to ageing, the process of becoming older. In industrialized nations the figure climbs to 90 percent. Ageing is among the greatest known risk factors for most human diseases. Yet not every living thing is bound by it. Bacteria, perennial plants, and some simple animals are potentially biologically immortal. Strawberry plants send out runners to clone themselves. Animals in the genus Hydra regenerate so well that they avoid dying of old age. So what is it that separates a human body, which wears down and fails, from a creature that does not? Why does the body break in some places before others, and what exactly accumulates inside us over a lifetime? The answers reach from a single yeast cell to the global economics of an ageing world.

• 2. Death And The Disposable Body

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  Early life forms on Earth, starting at least 3.7 billion years ago, were single-celled organisms. Prokaryotes, protozoans, and algae multiply by fission into daughter cells, which led scientists to think such organisms do not age and are potentially immortal under favorable conditions. That clean picture has since frayed. Evidence has been reported that ageing leading to death occurs even in the bacterium Escherichia coli, which reproduces by morphologically symmetrical division. Ageing has also been reported in the bacterium Caulobacter crescentus and the single-cell yeast Saccharomyces cerevisiae.

  The mortality of the individual became more evident with the evolution of eukaryotic sexual reproduction. This emerged with the fungal and animal kingdoms roughly a billion years ago, and with seed-producing plants 320 million years ago. A sexual organism could pass some genetic material on to new individuals, then become disposable with respect to its species' survival. The discovery that E. coli may split into distinguishable daughter cells has perturbed that classic idea, opening the theoretical possibility of "age classes" among bacteria.

  Immortality still hides inside mortal bodies. Cancer cells lose the ability to die when maintained in culture, as seen in the HeLa cell line. Germ cells, which produce ova and spermatozoa, carry the same potential. In artificial cloning, adult cells can be rejuvenated to embryonic status and grown into new tissue without ageing. Normal human cells, by contrast, die after about 50 cell divisions in laboratory culture. That ceiling is the Hayflick limit, discovered by Leonard Hayflick in 1961.

• 3. A Body Wearing Down

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  Teenagers tend to lose the young child's ability to hear high-frequency sounds above 20 kHz. From there the timeline of decline unfolds with surprising regularity. After age 35, female fertility falls rapidly. Most people experience presbyopia, a hardening of the eye's lens, by age 45 to 50. By age 55, hair tends to grey, and pattern hair loss by that age affects about 30 to 50 percent of males and a quarter of females. Menopause typically occurs between 44 and 58 years of age.

  In the 60 to 64 age cohort, the incidence of osteoarthritis rises to 53 percent, though only 20 percent report disabling osteoarthritis. Almost half of people older than 75 have hearing loss, known as presbycusis, that inhibits spoken communication. Here is a strange evolutionary footnote: fish, birds, and amphibians do not develop presbycusis, because they regenerate their cochlear sensory cells. Mammals, including humans, have genetically lost that ability. By age 80, more than half of all Americans either have a cataract or have had cataract surgery.

  Frailty, a syndrome of decreased strength, physical activity, performance, and energy, affects 25 percent of those over 85. Muscles lose mass and strength in a process called sarcopenia, and maximum oxygen use and heart rate decline. Atherosclerosis, classified as an ageing disease, leads to cardiovascular disease, the most common cause of death globally. The maximum human lifespan is suggested to be around 115 years. The oldest reliably recorded human was Jeanne Calment, who died in 1997 at 122 years of age.

• 4. The Mind Across The Years

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  About 3 percent of people between the ages of 65 and 74 have dementia, rising to 19 percent between 75 and 84, and nearly half of those over 85. The spectrum runs from mild cognitive impairment to neurodegenerative diseases including Alzheimer's, cerebrovascular disease, Parkinson's, and Lou Gehrig's disease. Behind some of this lies a physical change: after age 20, the total length of the brain's myelinated axons drops by 10 percent each decade.

  Not everything fades. Many types of memory decline with age, but semantic memory and general knowledge such as vocabulary definitions do not. These typically increase or stay steady until late adulthood. Intelligence may in fact remain steady throughout most of the human lifespan, dropping suddenly only as people near the end of their lives. That pattern suggests individual variation in cognitive decline may be explained by people simply having different lengths of life.

  The emotional picture defies expectation too. Visual impairment can reduce non-verbal communication, leading to isolation and possible depression. Macular degeneration affects nearly 12 percent of those above the age of 80. Yet older adults may not experience depression as much as younger adults, and were paradoxically found to have improved mood despite declining physical health. That paradox returns in how people see themselves, where perception and biology diverge even further.

• 5. Why We Age At All

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  Life span, like other phenotypes, is selected for in evolution. The logic is unsettling: traits that benefit early survival and reproduction will be selected for even if they contribute to an earlier death. When a single gene carries that double function, enabling reproduction young while costing life expectancy in old age, it is called the antagonistic pleiotropy effect. When an entire genetic programme diverts limited resources from maintenance to reproduction, it is called the disposable soma effect.

  The factors that drive biological ageing fall into two main categories, programmed and error-related. Programmed factors follow a biological timetable, possibly a continuation of the inherent mechanisms that regulate childhood growth and development. The rate of ageing varies substantially across species, and to a large extent this is genetically based. In 2008 it was discovered that inactivating only two genes in the annual plant Arabidopsis thaliana converts it into a potentially immortal perennial plant.

  Some lifespans dwarf our own. The oldest animals known so far are 15,000-year-old Antarctic sponges. The bristlecone pine reaches 5062 or 5067 years. The hard clam, known as quahog in New England, reaches 508 years, the Greenland shark 400 years, and deep-sea tube worms over 300 years. Such organisms are sometimes said to exhibit negligible senescence. The genetic aspect has also been demonstrated in studies of human centenarians, hinting that the timetable can run very differently from one body to the next.

• 6. The Damage That Accumulates

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  DNA damage is thought to be the common basis of both cancer and ageing. It has been argued that intrinsic causes of DNA damage are the most important causes of ageing. Genetic damage, mutations, and epimutations can all cause abnormal gene expression, forcing cells to stop dividing or undergo apoptosis, often draining stem cell pools and hindering regeneration. A meta-analysis of 36 studies with 4,676 participants showed an association between age and DNA damage in humans. In healthy humans after age 50, chronological age shows a linear association with DNA damage accumulation in blood mononuclear cells.

  The comparison between species is striking. Dogs annually lose approximately 3.3 percent of the DNA in their heart muscle cells, while humans lose approximately 0.6 percent each year. Those figures sit close to the ratio of the two species' maximum longevities, 120 years against 20 years, a 6 to 1 ratio. Lead author Bernard L. Strehler stated that "genetic damage (particularly gene loss) is almost certainly (or probably the) central cause of ageing."

  Other theories point to buildup rather than loss. A waste product called lipofuscin, formed when fat binds to proteins, may accumulate as small granules during ageing. The free-radical theory blames reactive oxygen species and oxidative stress. Supporting it, caloric restriction reduces 8-OH-dG DNA damage in organs of ageing rats and mice. In a 2021 review article, Vijg stated that "Based on an abundance of evidence, DNA damage is now considered as the single most important driver of the degenerative processes that collectively cause ageing."

• 7. The Midlife Cliff

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  A 2025 study published in Cell examined protein changes in 516 tissue samples collected from 76 human donors ranging in age from 14 to 68. Researchers built tissue-specific "proteomic clocks" to trace how ageing progresses across different organs. Their analysis revealed a noticeable shift in the rate of ageing around age 50, with the steepest changes occurring between ages 45 and 55, particularly in blood vessels.

  Among the tissues studied, the aorta showed the most significant changes in protein makeup. One identified protein, GAS6, was linked to faster ageing; when introduced into mice, it triggered early vascular deterioration and other signs of systemic ageing. The research also found that some organs, like the adrenal gland, begin showing ageing-related changes as early as age 30. The findings suggest human ageing accelerates midlife rather than progressing steadily, which could help guide earlier and more targeted approaches to support healthier ageing.

• 8. An Older World

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  Over 11 percent of the world's current population are people aged 60 and older, and the United Nations Population Fund estimates that by 2050 that number will rise to approximately 22 percent. This shift came from development that enabled better nutrition, sanitation, health care, education, and economic well-being. Fertility rates fell and life expectancy rose. Life expectancy at birth is now over 80 in 33 countries. The trend is occurring fastest in developing countries, including those with large youth populations.

  The economics are heavy. Ageism cost the United States 63 billion dollars in one year, according to a Yale School of Public Health study. Population ageing only explains 0.2 percentage points of the annual growth rate in medical spending of 4.3 percent since 1970, with rising incomes and costly new technology doing most of the work. The unpaid contributions of older people are large too. A 2003 Australian Government study estimated that women aged 65 to 74 contribute 16 billion Australian dollars per year in unpaid caregiving and voluntary work, and men in the same group 10 billion dollars.

  Not every society treats age the same way. In Korea, a special party called hwangap celebrates an individual turning 60 years old. In China, respect for elders has been at the foundation of morality for thousands of years. The way people see their own ageing carries weight as well. As people age, subjective health stays relatively stable even as objective health worsens, a phenomenon known as the "paradox of ageing." To let the young feel what the old feel, designers built ageing suits such as the GERT, the R70i exoskeleton, and the AGNES, or Age Gain Now Empathy Suit, which add weight and pressure to the joints and impair vision and hearing so that a person can wear old age for an afternoon.