Eternal youth

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. Eternal youth is a characteristic of the inhabitants of Paradise in Abrahamic religions. 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 siddhas 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 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 apples that grant them eternal youthfulness in the 13th-century Prose Edda. An individual's DNA plays a role in the aging process. Aging begins even before birth, as soon as cells start to die and need to be replaced. On the ends of each chromosome are repetitive sequences of DNA, telomeres, that protect the chromosome from joining with other chromosomes, and have several key roles. One of these roles is to regulate cell division by allowing each cell division to remove a small amount of genetic code. The amount removed varies by the cell type being replicated. The gradual degradation of the telomeres restricts cell division to 40-60 times, also known as the Hayflick limit. Once this limit has been reached, more cells die than can be replaced in the same time span. Thus, soon after this limit is reached the organism dies. The importance of telomeres is now clearly evident: lengthen the telomeres, lengthen the life.