Ageing

Ageing (or aging in American English) describes the process of getting older. While the term mainly applies to humans, many other animals, and fungi, some life forms—such as bacteria, perennial plants, and certain simple animals—are considered potentially biologically immortal. More broadly, ageing can refer to individual cells within an organism that have stopped dividing, or to the overall population of a species. In humans, ageing involves the buildup of changes over time, affecting physical, psychological, and social aspects. For instance, reaction time may slow, while memory and general knowledge often increase. Globally, about 150,000 people die each day, and roughly two-thirds of those deaths stem from age-related causes.

Current theories on ageing fall into two main categories. The damage concept holds that accumulated harm—like DNA oxidation—causes biological systems to fail. The programmed ageing concept suggests that internal processes, such as epigenetic maintenance like DNA methylation, inherently drive ageing. Programmed ageing is distinct from programmed cell death, or apoptosis.

**Ageing versus immortality**

Humans and many other species, including animals and fungi, age and die. In contrast, some species are potentially immortal: bacteria reproduce by fission, creating daughter cells; strawberry plants send out runners to form clones; and animals in the genus *Hydra* can regenerate to avoid dying of old age. Early life on Earth, appearing at least 3.7 billion years ago, consisted of single-celled organisms like prokaryotes, protozoans, and algae, which multiply by fission. These were long thought not to age and to be potentially immortal under good conditions. However, evidence shows that the bacterium *Escherichia coli*, which reproduces through morphologically symmetrical division, can age and die. Similar ageing has been observed in the bacterium *Caulobacter crescentus* and the single-celled yeast *Saccharomyces cerevisiae*.

Ageing and individual mortality became more pronounced with the evolution of eukaryotic sexual reproduction, which emerged about a billion years ago with the fungal and animal kingdoms, and later with seed-producing plants around 320 million years ago. Sexual organisms could pass genetic material to new individuals, making the parent organism potentially disposable for the species’ survival. This classic idea has been complicated by the discovery that *E. coli* can split into distinguishable daughter cells, hinting at possible “age classes” among bacteria.

Even within humans and other mortal species, some cells are potentially immortal. Cancer cells, like those in the HeLa cell line, can lose the ability to die in culture. Specific stem cells, such as germ cells that produce ova and spermatozoa, also show this potential. In artificial cloning, adult cells can be rejuvenated to an embryonic state and used to grow new tissue or animals without ageing. Normal human cells, however, die after about 50 divisions in lab culture—a limit known as the Hayflick limit, discovered by Leonard Hayflick in 1961.

**Signs and symptoms**

Many characteristic signs of ageing affect a majority or significant portion of humans, though the changes vary across bodily systems and individuals—a pattern called mosaic ageing.

- Teenagers typically lose the ability to hear high-frequency sounds above 20 kHz, a capacity young children have. - Wrinkles develop mainly from photoageing, especially on sun-exposed areas like the face. - Female fertility drops sharply after age 35. - Most people develop presbyopia by ages 45–50, caused by lens hardening from decreasing levels of alpha-crystallin (a process accelerated by higher temperatures) or by loss of strength in the ciliary muscle. - Hair tends to grey by age 55. - Pattern hair loss by age 55 affects about 30–50% of males and a quarter of females. - Menopause typically occurs between ages 44 and 58. - In the 60–64 age group, osteoarthritis incidence rises to 53%, though only 20% report disabling symptoms. - Nearly half of people over 75 have hearing loss (presbycusis) that hinders spoken communication. Many vertebrates like fish, birds, and amphibians can regenerate cochlear sensory cells and avoid presbycusis, but mammals, including humans, have lost this ability genetically. - By age 80, more than half of Americans either have a cataract or have had cataract surgery. - Frailty—a syndrome of decreased strength, physical activity, performance, and energy—affects 25% of those over 85. Muscles respond less to exercise or injury, and loss of muscle mass and strength (sarcopenia) is common. Maximum oxygen use and heart rate decline, and hand strength and mobility decrease. - Atherosclerosis is considered an ageing disease, leading to cardiovascular issues like stroke and heart attacks, the most common cause of death globally. Vessel ageing causes vascular remodelling and loss of arterial elasticity, resulting in stiffened vasculature. - The maximum human lifespan is thought to be around 115 years. The oldest reliably recorded human was Jeanne Calment, who died in 1997 at age 122. - Dementia becomes more common with age: about 3% of people aged 65–74, 19% of those 75–84, and nearly half of those over 85 have dementia, ranging from mild cognitive impairment to severe forms.