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Eternal youth is the concept of human physical immortality free of ageing. The youth referred to is usually meant to be in contrast to the depredations of aging, rather than a specific age of the human lifespan. Eternal youth is common in mythology, and is a popular theme in fiction.
The Hindus believe that the Vedic and the post-Vedic rishis have attained immortality, which implies the ability to change one's body's age or even shape at will. These are some of the in Yoga. Markandeya is said to always stay at the age of 16.
The difference between eternal life and the more specific eternal youth is a recurrent theme in Greek mythology and Roman mythology. The mytheme of requesting the boon of immortality from a god, but forgetting to ask for eternal youth appears in the story of Tithonus. A similar theme is found in Ovid regarding the Cumaean Sibyl.
In Norse mythology, Iðunn is described as providing the gods that grant them eternal youthfulness in the 13th-century Prose Edda.
However, a study of the comparative biology of mammalian telomeres indicated that telomere length correlates inversely, rather than directly, with lifespan, and concluded that the contribution of telomere length to lifespan remains controversial. Also, telomere shortening does not occur with age in some postmitotic tissues, such as in the rat brain. In humans, skeletal muscle telomere lengths remain stable from ages 23–74. In baboon skeletal muscle, that consists of fully differentiated post-mitotic cells, less than 3% of myonuclei contain damaged telomeres and this percentage does not increase with age. Thus telomere shortening does not appear to be a major factor in the aging of the differentiated cells of brain or skeletal muscle.
Studies have shown that 90 percent of cancer cells contain large amounts of an enzyme called telomerase. Telomerase is an enzyme that replenishes the worn away telomeres by adding bases to the ends and thus renewing the telomere. A cancer cell has in essence turned on the telomerase gene, and this allows them to have an unlimited amount of divisions without the telomeres wearing away. Other kinds of cells that can surpass the Hayflick limit are stem cells, hair follicles, and germ cells. This is because they contain raised amounts of telomerase.
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