When age increase senescence occurs .which means impairment of physiological functions. The senescence decreases the tolerance power of body against various diseases when age increases the human growth hormone level declined. The production of sex hormones like estrogens and testosterone decreased.

The sex hormone present in female namely estrogen  slows the bone thinning and may help to resist frilly and disability . the hormone melatonin regulates seasonal changes in the body . the decline level of melatonin causes changes in endocrime system. The homone dehydroepiandrosterone level declines which cause ageing and power of immune system also declined. The programmed senescence theory associated with ageing says that ageing is a result of sequential switching on and off of certain genes . another theory related with ageing is damage or error theories , it says that the aspects of rate of living , free radicles , wear and tear , somatic mutations causes ageing . ageing affects all living body. Ageing is a deleterious process .when we age the amount of stress required  to cause a breakdown in the organisms health falls.