Ageing/ Aging is the process of becoming older. Ageing can refer to single cells within an organism which have ceased dividing (cellular senescence) or tothe population of a species (population ageing). Aging occurs due to accumulation of physical, psychological, and social changes in a human being over time. Ageing is one of the risk factors for most human diseases. Approximately two thirds of the 150,000 of total deaths occurring globally per day is due to age related factors. Aging is characterized by a progressive, generalized impairment of function, resulting in an increasing vulnerability to environmental challenge and a growing risk of disease and death. Ageing and mortality started with evolution of sexual reproduction and emergence of the fungal/animal kingdoms approximately a billion years ago. Normal human cells die after about 50 cell divisions known as Hayflick Limit. There are several theories postulated for the cellular mechanism of ageing. They are broadly divided into two main categories: Programmed and Damage­related. Cellular senescence has been attributed to the shortening of telomeres at each cell division. DNA damage theory forms the common basis of both cancer and ageing. Intrinsic causes of DNA damage are the most important drivers of ageing. According to Free radical theory oxidative reaction leads to the formation of molecular species with unpaired electrons which makes them highly reactive known as Free radicals. Ageing affects the various systems of our body. Prevention and delay of ageing can be done by lifestyle modification. The amount of sleep has great impact on mortality. Physical exercises may also increase life expectancy. Avoidance of chronic stress slows loss of telomeres and decreases cortisol levels. Caloric restriction also delays ageing. 

Keywords: Ageing; Hayflick Limit; Telomere; Free Radicals; Caloric Restriction.