There has been significant debate among demographers as to what happens to the rate of mortality at extreme old ages. The answer has important ramifications for deciphering the factors that are important to survival to extreme old age. An important possibility is that demographic selection occurs at very old age, yielding a select, relatively healthy cohort at even older ages. Demographic selection describes the process of old frail individuals dying, leaving behind a more fit surviving cohort (Vaupel et al., 1998). The observation by some research groups that the incidence of AD plateaus at very old ages is consistent with this phenomenon. Ritchie and Kildea (1995) performed a meta-analysis of nine epide-miological studies, finding that the rate of increase in dementia prevalence fell off among octogenarians and plateaued at approximately 40% at age 95. In a longitudinal study of older people living in Cache County, Utah, the incidence of both dementia and AD increased almost exponentially until ages 85 to 90, but declined after age 93 for men and age 97 for women (Miech, 2002).
A genetic example of demographic selection is the decreased frequency of the apolipoprotein E epsilon-4 (ApoE fi-4) allotype in the oldest old. Individuals who are homozygous for ApoE e-4 have 2.3 to 8.0 times greater risk of developing AD than the general Caucasian population (Corder, 1993). The allelic frequency of ApoE e-4 drops off dramatically in the oldest age groups, presumably because of its association with AD and vascular disease (Schachter et al., 1994). Interestingly, the effect of ApoE allotype upon AD incidence appears to decrease with age at these very old ages (Sobel, 1995).
These examples notwithstanding, the phenomenon of demographic selection occurring at extreme old ages remains controversial. Some argue that because of marked improvements in medical care and public health measures, such selection no longer occurs. In this case, decreased mortality due to improved health care could lead to an increased prevalence of frailty among older survivors because treatment of existing diseases simply postpones death to older ages (Carnes, 2001).
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