HALDANE, MEDAWAR, AND WILLIAMS argued, “aging evolves because natural selection becomes inefficient at maintaining function (and fitness) at old age. Their ideas were later mathematically formalized by William D. Hamilton and Brian Charlesworth in the 1960’s and 1970’s, and today they are empirically well supported.”
http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151