The period from birth to the natural death of an organism is called its life span.
Life span of an organism may be few minutes to several thousand years. Life span of Mayfly is one day, while giant tortoise is considered the longest living animal (about 100 to 150 years).
Life span of an organism usually includes four stages:
(i) Juvenility- During this stage organism develops the capacity to reproduce.
(ii) Maturity- Reproduction begins during this stage
(iii) Ageing and Senescence- Ageing is progressive deterioration in the body of the organisms. The terminal irreversible stage of ageing is called senescence.
(iv) Death- Senescence finally leads to death. During this stage there is final permanent cessation of all vital activities of an organism.
It means no individual is immortal except some single-celled (e.g., Amoeba) organisms.