For the purposes of his work into aging and longevity, researcher Michael Rose defines the condition of mortality as being a rising risk of death as time goes on. This is what we see in the world we know: the older a person, a pet, a plant becomes, the more likely it is to die in any given period of time. Immortality, it then follows, is the state of being in which the risk of death does not rise with advancing time.
As it turns out, our intuitions about the way in which aging and risk of death progress do not accurately reflect what happens in very late life - for those who live much longer than the average. Rose demonstrated that very old flies no longer suffer an increasing risk of death; even though the actual mortality rate is high, it stops growing. These are immortal flies by his definition.
This plateau of late-life immortality is expected to exist in all higher animals, and hints have been seen in human data in recent years:
Increased age is regularly linked with heightened cancer risk, but recent research suggests a flattening around age 80. We report that, independent of cancer site or time period, most incidence rates decrease in the more elderly and drop to or toward zero near the ceiling of human life span
The International Database of Longevity (IDL) offers detailed information on thoroughly validated cases of supercentenarians. These data are used to estimate human mortality after age 110. The procedure properly accounts for the country-specific sampling frames in the IDL. The analysis confirms that human mortality after age 110 is at at a level corresponding to an annual probability of death of 50%. No sex-specific differences in mortality could be found, and no time trend in supercentenarian mortality between earlier and later cohorts could be detected.
This should not be too surprising, and is a result expected by many researchers. The immortality plateau behavior might be thought to be a fundamental property of the aging and failure of complex redundant systems - such as us. As immortality goes, it's a pretty raw deal, of course. Our job in the here and now is to work hard at developing the near-term biotechnology of rejuvenation, so that we never have to experience late life immortality the natural way.