It might seem too simplistic to equate old people with old cars, but the comparison is a valid one. Everything ages. It is the accumulation of damage that we incur simply by being here. As with cars, our bodies break down with time, not because of any manufacturing defect, but due to wear and tear. The damage suffered by our biological machines over the years, makes them liable to break down even at normal, moderate levels of use. Such appliances are simply old and in need of repair.