My favorite explanation of Againg comes from William Hamilton. He took advantage of the Euler-Lotka equation to calculate the number of offspring that would be alive at any given point in the future. Using this equation from the field of demographics, he showed that senescence has only a marginal effect on fitness. As such, selective pressure would not be strong enough to filter out mutations that might result in aging. Finally, he was able to derive various mortality curves that "seem" to match pretty well with real living systems.