Ageing

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.
In humans, ageing represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes. 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.
Current ageing theories are assigned to the damage concept, whereby the accumulation of damage may cause biological systems to fail, or to the programmed ageing concept, whereby the internal processes inherently may cause ageing. Programmed ageing should not be confused with programmed cell death.

Ageing versus immortality

Human beings and members of other species, especially animals, age and die. Fungi, too, can age. 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, were single-celled organisms. Such organisms multiply by fission into daughter cells; thus single celled organisms have been thought to not age and to be potentially immortal under favorable conditions. 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. Evidence of aging has also been reported for the bacterium Caulobacter crescintus. and the single cell yeast Saccharomyces cerevisiae.
Ageing and mortality of the individual organism became more evident with the evolution of eukaryotic sexual reproduction, 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. 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.
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, and specific stem cells such as germ cells. In artificial cloning, adult cells can be rejuvenated to embryonic status and then used to grow a new tissue or animal without ageing. Normal human cells however die after about 50 cell divisions in laboratory culture.

Symptoms

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.
Wrinkles develop mainly due to photoageing, particularly affecting sun-exposed areas such as the face.
After peaking from the late teens to the late 20s, female fertility declines.
After age 30, the mass of the human body is decreased until 70 years and then shows damping oscillations.
People over 35 years of age are at increasing risk for losing strength in the ciliary muscle of the eyes, which leads to difficulty focusing on close objects, or presbyopia. Most people experience presbyopia by age 45–50. The cause is lens hardening by decreasing levels of alpha-crystallin, a process which may be sped up by higher temperatures.
By age 55, hair tends to grey. Pattern hair loss by the age of 55 affects about 30–50% of males and a quarter of females.
Menopause typically occurs between 44 and 58 years of age.
In the 60–64 age cohort, the incidence of osteoarthritis rises to 53%. Only 20%, however, report disabling osteoarthritis at this age.
Almost half of people older than 75 have hearing loss, inhibiting spoken communication. 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.
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, physical performance and energy, affects 25% of those over 85. Muscles have a reduced capacity of responding to exercise or injury and loss of muscle mass and strength is common. Maximum oxygen use and maximum heart rate decline. Hand strength and mobility decrease.
Atherosclerosis is classified as an ageing disease. It leads to cardiovascular disease, which, globally, is the most common cause of death. Vessel ageing causes vascular remodelling and loss of arterial elasticity, and as a result, causes the stiffness of the vasculature.
The heart undergoes structural and functional changes with aging including an increase in left ventricular mass, which eventually may lead to heart failure.
Evidence suggests that age-related risk of death plateaus after the age of 105. The maximum human lifespan is suggested to be around 115 years although it was proposed previously to be 125. The oldest reliably recorded human was Jeanne Calment, who died in 1997 at 122 years of age.

Dementia becomes more common with age. 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. 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 . 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. 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.
Age can result in visual impairment, whereby non-verbal communication is reduced, 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. Macular degeneration causes vision loss and increases with age, affecting nearly 12% of those above the age of 80. This degeneration is caused by systemic changes in the circulation of waste products and by the growth of abnormal vessels around the retina.
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. 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. 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.
In addition to physical symptoms, aging can also cause a number of mental health issues as older adults deal with challenges such as the death of loved ones, retirement and loss of purpose, as well as their own health issues. Some warning signs are: changes in mood or energy, changes in sleep or eating habits, pain, sadness, unhealthy coping mechanisms such as smoking, suicidal ideations, and others. Older adults are more prone to social isolation as well, which can further increase the risk for physical and mental conditions such as anxiety, depression, and cognitive decline.
A distinction can be made between "proximal ageing" and "distal ageing".
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. In industrialized nations, the proportion is higher, reaching 90%.

Biological basis

In 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. Little is known of mammalian ageing, in part due to the much longer lives of even small mammals, such as the mouse. 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. Most known mutations and RNA interference targets that extend lifespan were first discovered in C. elegans.
The factors proposed to influence biological ageing fall into two main categories, programmed and error-related. Programmed factors follow a biological timetable that might be a continuation of inherent mechanisms that regulate childhood growth and development. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses.
Factors causing errors or damage include internal and environmental events that induce cumulative deterioration in one or more organs.