Evolutionary scientists have been trying to form a consolidated theory of ageing since the 1890's.
Enquiry into the evolution of ageing aims to explain why a detrimental process such as ageing would evolve, and why there is so much variability in the lifespans of living organisms. The classical theories of evolution (mutation accumulation, antagonistic pleiotropy, and disposable soma) suggest that environmental factors such as predation, accidents, disease, starvation, etc. ensure that most organisms living in natural settings will not live until old age, and so there will be very little pressure to conserve genetic changes that increase longevity. Natural selection will instead strongly favor genes which ensure early maturation and rapid reproduction, and the selection for genetic traits which promote molecular and cellular self-maintenance will decline with age for most organisms. Source: Wikipedia
One theory is that random deleterious mutations can be minimised if lifeforms have limited lifespans, but no theory yet explains how mutations can affect fitness at different ages and the evolution of senescence.
The huge variability of lifespans is also unexplained. There are many species which appear to have no increase in mortality after maturity - so-called 'Negligible Senescence' - e.g. in fish (sturgeon) , plants (aspen), arthropods (lobsters) etc etc.