Ageing is the process of becoming older. The term refers mainly to humans, many other animals, and fungi; whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In a broader sense, ageing can refer to single cells within an organism which have ceased dividing, or to the population of a species.
Ageing (or aging in American English) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi; whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In a broader sense, ageing can refer to single cells within an organism which have ceased dividing, or to the population of a species.[1]
In humans, ageing represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes.[2][3] Reaction time, for example, may slow with age, while memories and general knowledge typically increase. Of the roughly 150,000 people who die each day across the globe, about two-thirds die from age-related causes.[4]
Current ageing theories are assigned to the damage concept, whereby the accumulation of damage (such as DNA oxidation) may cause biological systems to fail, or to the programmed ageing concept, whereby the internal processes (epigenetic maintenance such as DNA methylation)[5] inherently may cause ageing. Programmed ageing should not be confused with programmed cell death (apoptosis).
Ageing versus immortality
Human beings and members of other species, especially animals, age and die. Fungi, too, can age.[6] In contrast, many species can be considered potentially immortal: for example, bacteria fission to produce daughter cells, strawberry plants grow runners to produce clones of themselves, and animals in the genus Hydra have a regenerative ability by which they avoid dying of old age.
Early life forms on Earth, starting at least 3.7 billion years ago,[7] were single-celled organisms. Such organisms (Prokaryotes, Protozoans, algae) multiply by fission into daughter cells; thus single celled organisms have been thought to not age and to be potentially immortal under favorable conditions.[8][9] However, evidence has been reported that aging leading to death occurs in the single-cell bacterium Escherichia coli, an organism that reproduces by morphologically symmetrical division.[10] Evidence of aging has also been reported for the bacterium Caulobacter crescintus.[11] and the single cell yeast Saccharomyces cerevisiae.[12][13]
Ageing and mortality of the individual organism became more evident with the evolution of eukaryotic sexual reproduction,[14] which occurred with the emergence of the fungal/animal kingdoms approximately a billion years ago, and the evolution of seed-producing plants 320 million years ago. The sexual organism could henceforth pass on some of its genetic material to produce new individuals and could itself become disposable with respect to the survival of its species.[14] This classic biological idea has however been perturbed recently by the discovery that the bacterium E. coli may split into distinguishable daughter cells, which opens the theoretical possibility of "age classes" among bacteria.[10]
Even within humans and other mortal species, there are cells with the potential for immortality: cancer cells which have lost the ability to die when maintained in a cell culture such as the HeLa cell line,[15] and specific stem cells such as germ cells (producing ova and spermatozoa).[16] In artificial cloning, adult cells can be rejuvenated to embryonic status and then used to grow a new tissue or animal without ageing.[17] Normal human cells however die after about 50 cell divisions in laboratory culture (the Hayflick limit, discovered by Leonard Hayflick in 1961).[15]
Symptoms
See also: Old age § Signs Hearing loss with age (presbycusis) Teenagers begin to lose the ability to hear high-pitched sounds.[18] Beyond the age of 25, many adults cannot hear this 10-second audio clip at a frequency of 17.4 kHz.[19] Problems playing this file? See media help.A number of characteristic ageing symptoms are experienced by a majority, or by a significant proportion of humans during their lifetimes.
- Teenagers tend to lose the young child's ability to hear high-frequency sounds above 20 kHz.[18]
- Wrinkles develop mainly due to photoageing, particularly affecting sun-exposed areas such as the face.[21]
- After age 35, female fertility falls rapidly.[22]
- Most people experience presbyopia by age 45–50.[23] The cause is lens hardening by decreasing levels of alpha-crystallin, a process which may be sped up by higher temperatures.[23][24] An alternative theory is loss of strength in the ciliary muscle of the eyes.[25][26]
- By age 55, hair tends to grey.[27]
- Pattern hair loss by the age of 55 affects about 30–50% of males[28] and a quarter of females.[29]
- Menopause typically occurs between 44 and 58 years of age.[30]
- In the 60–64 age cohort, the incidence of osteoarthritis rises to 53%. Only 20%, however, report disabling osteoarthritis at this age.[31]
- Almost half of people older than 75 have hearing loss (presbycusis) inhibiting spoken communication.[32] Many vertebrates such as fish, birds and amphibians do not develop presbycusis in old age, as they are able to regenerate their cochlear sensory cells; mammals, including humans, have genetically lost this ability.[33]
- By age 80, more than half of all Americans either have a cataract or have had cataract surgery.[34]
- Frailty, a syndrome of decreased strength, physical activity, physical performance and energy, affects 25% of those over 85.[35][36] Muscles have a reduced capacity of responding to exercise or injury and loss of muscle mass and strength (sarcopenia) is common.[37] Maximum oxygen use and maximum heart rate decline.[38] Hand strength and mobility decrease.[39]
- Atherosclerosis is classified as an ageing disease.[40] It leads to cardiovascular disease (for example, stroke and heart attacks),[41] which, globally, is the most common cause of death.[42] Vessel ageing causes vascular remodelling and loss of arterial elasticity, and as a result, causes the stiffness of the vasculature.[40]
- The maximum human lifespan is suggested to be around 115 years.[43][44] The oldest reliably recorded human was Jeanne Calment, who died in 1997 at 122 years of age.
Dementia becomes more common with age.[45] About 3% of people between the ages of 65 and 74, 19% of those between 75 and 84, and nearly half of those over 85 years old have dementia.[46] The spectrum ranges from mild cognitive impairment to the neurodegenerative diseases of Alzheimer's disease, cerebrovascular disease, Parkinson's disease and Lou Gehrig's disease. Furthermore, many types of memory decline with ageing, but not semantic memory or general knowledge such as vocabulary definitions. These typically increase or remain steady until late adulthood [47] (see: Ageing brain). Intelligence declines with age, though the rate varies depending on the type and may, in fact, remain steady throughout most of the human lifespan, dropping suddenly only as people near the end of their lives. Individual variations in the rate of cognitive decline may therefore be explained in terms of people having different lengths of life.[48] There are changes to the brain: after 20 years of age, there is a 10% reduction each decade in the total length of the brain's myelinated axons.[49][50]
Age can result in visual impairment, whereby non-verbal communication is reduced,[51] which can lead to isolation and possible depression. Older adults, however, may not experience depression as much as younger adults, and were paradoxically found to have improved mood, despite declining physical health.[52] Macular degeneration causes vision loss and increases with age, affecting nearly 12% of those above the age of 80.[53] This degeneration is caused by systemic changes in the circulation of waste products and by the growth of abnormal vessels around the retina.[54]
Other visual diseases that often appear with age are cataracts and glaucoma. A cataract occurs when the lens of the eye becomes cloudy, making vision blurry; it eventually causes blindness if untreated.[55] They develop over time and are seen most often with those that are older. Cataracts can be treated through surgery. Glaucoma is another common visual disease that appears in older adults. Glaucoma is caused by damage to the optic nerve, causing vision loss.[56] Glaucoma usually develops over time, but there are variations to glaucoma, and some have a sudden onset. There are a few procedures for glaucoma, but there is no cure or fix for the damage, once it has occurred. Prevention is the best measure in the case of glaucoma.[56]
A distinction can be made between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced to a cause in a person's early life, such as childhood poliomyelitis).[48]
Ageing is among the greatest known risk factors for most human diseases. Of the roughly 150,000 people who die each day across the globe, about two-thirds — 100,000 per day — die from age-related causes.[57] In industrialized nations, the proportion is higher, reaching 90%.[57][58][59]
Biological basis
Main article: SenescenceIn the 21st century, researchers are only beginning to investigate the biological basis of ageing even in relatively simple and short-lived organisms, such as yeast.[60] Little is known of mammalian ageing, in part due to the much longer lives of even small mammals, such as the mouse (around 3 years). A model organism for the study of ageing is the nematode C. elegans – having a short lifespan of 2–3 weeks – enabling genetic manipulations or suppression of gene activity with RNA interference, and other factors.[61] Most known mutations and RNA interference targets that extend lifespan were first discovered in C. elegans.[62]
The factors proposed to influence biological ageing fall into two main categories, programmed and error-related.[63] Programmed factors follow a biological timetable that might be a continuation of inherent mechanisms that regulate childhood growth and development.[63] This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses.[63] Factors causing errors or damage include internal and environmental events that induce cumulative deterioration in one or more organs.[63]
Programmed factors
The rate of ageing varies substantially across different species, and this, to a large extent, is genetically based. For example, numerous perennial plants ranging from strawberries and potatoes to willow trees typically produce clones of themselves by vegetative reproduction and are thus potentially immortal, while annual plants such as wheat and watermelons die each year and reproduce by sexual reproduction. In 2008 it was discovered that inactivation of only two genes in the annual plant Arabidopsis thaliana leads to its conversion into a potentially immortal perennial plant.[64] The oldest animals known so far are 15,000-year-old Antarctic sponges,[65] which can reproduce both sexually and clonally.
Clonal immortality apart, there are certain species whose individual lifespans stand out among Earth's life-forms, including the bristlecone pine at 5062 years[66] or 5067 years,[65] invertebrates like the hard clam (known as quahog in New England) at 508 years,[67] the Greenland shark at 400 years,[68] various deep-sea tube worms at over 300 years,[69] fish like the sturgeon and the rockfish, and the sea anemone[70] and lobster.[71][72] Such organisms are sometimes said to exhibit negligible senescence.[73] The genetic aspect has also been demonstrated in studies of human centenarians.
Midlife Acceleration
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 developed 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.[74][75][76]
Among the various tissues studied, the aorta showed the most significant changes in protein makeup. One of the proteins identified, GAS6, was linked to faster ageing; when introduced into mice, it triggered early vascular deterioration and other signs of systemic ageing.[77] The research also found that some organs, like the adrenal gland, begin showing ageing-related changes as early as age 30,[78] suggesting that not all parts of the body age at the same pace. The findings suggest that human ageing accelerates midlife rather than progressing steadily throughout adulthood. Understanding this shift could help guide earlier and more targeted approaches to support healthier ageing[79][80]
Evolution of ageing
Main article: Evolution of ageingLife span, like other phenotypes, is selected for in evolution. Traits that benefit early survival and reproduction will be selected for even if they contribute to an earlier death. Such a genetic effect is called the antagonistic pleiotropy effect when referring to a gene (pleiotropy signifying the gene has a double function – enabling reproduction at a young age but costing the organism life expectancy in old age) and is called the disposable soma effect when referring to an entire genetic programme (the organism diverting limited resources from maintenance to reproduction).[14] The biological mechanisms which regulate lifespan probably evolved with the first multicellular organisms more than a billion years ago.[62] However, even single-celled organisms such as yeast have been used as models in ageing; hence, ageing has its biological roots much earlier than multi-cellularity.[81]
