Short answers, pulled from the story.
Ageing is the process of becoming older, referring mainly to humans, many other animals, and fungi. In humans it represents the accumulation of physical, psychological, and social changes over time. Bacteria, perennial plants, and some simple animals are by contrast potentially biologically immortal.
Of the roughly 150,000 people who die each day across the globe, about two-thirds, around 100,000 per day, die from age-related causes. In industrialized nations the proportion reaches 90 percent. Ageing is among the greatest known risk factors for most human diseases.
The Hayflick limit is the point at which normal human cells die after about 50 cell divisions in laboratory culture. It was discovered by Leonard Hayflick in 1961. Cancer cells and certain stem cells can bypass this limit and continue dividing.
A 2025 study published in Cell examined protein changes in 516 tissue samples from 76 human donors aged 14 to 68 and built tissue-specific proteomic clocks. It found ageing accelerates midlife, with the steepest changes between ages 45 and 55, especially in blood vessels. The aorta showed the most significant protein changes, and the protein GAS6 was linked to faster ageing.
Humans, other animals, and fungi age and die, while many species are potentially immortal because they reproduce by cloning or fission, such as bacteria, strawberry plants, and Hydra. Mortality became more evident with the evolution of sexual reproduction roughly a billion years ago, when organisms could pass on genetic material and become disposable for their species' survival.
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.
DNA damage is thought to be the common basis of both cancer and ageing, with intrinsic causes of DNA damage argued to be the most important causes of ageing. Dogs lose about 3.3 percent of heart muscle cell DNA per year while humans lose about 0.6 percent, close to the 6 to 1 ratio of their maximum longevities. In a 2021 review, Vijg called DNA damage the single most important driver of the degenerative processes that cause ageing.