Aging is a natural symptom which starts after the twenty years old. There are different theories explaining the mechanism of aging in the human being:

• the physical decline theory
• the neuro-endocrinal theory
• the biological-clock theory
• the hormone of death theory
• teoria della riparazione dei danni del DNA
• the repairing of DNA damages theory
• the theory of autoimmunity
• the theory of the pre-arranged number of cellular separation
• the loss of telomeres theory and the theory of oxidative damage caused by Free Radicals.