That aging is programmed is a minority view at present. Most researchers consider it to be a process of accumulated damage. As to why aging exists in multicell

That aging is programmed is a minority view at present. Most researchers consider it to be a process of accumulated damage. As to why aging exists in multicellular organisms, the evolutionary explanations tend to work whether or not aging is a matter of failure to evolve longevity-assurance mechanisms because selection pressure is low in post-reproductive life, failure to restrict selection of enhancements in youth that go on to cause harm in old age because selection pressure is low in post-reproductive life (such as an adaptive immune system that kills pathogens very efficiently in youth, but which goes haywire sometimes to attack the owners tissues, and suffers structural failure after exposure to too many pathogens), or a matter of the selected evolution of death programs. A compelling explanation is that species that age most likely outcompete those that do not during periods of environmental change. http://dx.doi.org/10.1371/journal.pone.0024328 http://dx.doi.org/10.1371/journal.pone.0024328