Calorie restriction, or caloric restriction, or energy restriction, is a dietary regimen that reduces calorie intake without incurring malnutrition or a reduction in essential nutrients. 'Low' can be defined relative to the subject's previous intake before intentionally restricting calories, or relative to an average person of similar body type. In a number of species, including yeast, fish, rodents and dogs, calorie restriction without malnutrition has been shown to slow the biological aging process, resulting in longer maintenance of youthful health and an increase in both median and maximum lifespan."
Source : Wikipedia

Detailed experimental research on caloric restriction's apparent ability to slow the ageing process has been going on for over 70 years, but "the mechanism underlying this phenomenon is still unknown." Source : Caloric Restriction and Aging: Studies in Mice and Monkeys

Theories regarding possible mechanisms include the effects of: • reduced core body temperature • reduced cellular divisions • lower metabolic rates • reduced production of free radicals • reduced DNA damage • hormesis (any process in a cell or organism that exhibits a biphasic response to exposure to increasing amounts of a substance or condition)