Aging is no more than damage at the level of cells and tissues and the evolved reactions of biological systems to that damage, not all of them helpful. The pace of damage accumulation is largely determined by lifestyle and environmental factors such as burden of infectious disease and available medical technology over most of a human life span. Only in very old age do common genetic differences rise in importance. Thus if you find that someone at a given chronological age is more frail and is suffering from more evident age-related conditions than their peers, you would expect them to have a shorter remaining life expectancy, since they are more damaged. That is the way it works:
We analyze life expectancy in Medicare beneficiaries by number of chronic conditions [in a] retrospective cohort study using single-decrement period life tables. [The subjects are] Medicare fee-for-service beneficiaries (N=1,372,272) aged 67 and older as of January 1, 2008.
Our primary outcome measure is life expectancy. We categorize study subjects by sex, race, selected chronic conditions (heart disease, cancer, chronic obstructive pulmonary disease, stroke, and Alzheimer disease), and number of comorbid conditions. Comorbidity was measured as a count of conditions collected by Chronic Conditions Warehouse and the Charlson Comorbidity Index.
Life expectancy decreases with each additional chronic condition. A 67-year-old individual with no chronic conditions will live on average 22.6 additional years. A 67-year-old individual with 5 chronic conditions and ≥10 chronic conditions will live 7.7 fewer years and 17.6 fewer years, respectively. The average marginal decline in life expectancy is 1.8 years with each additional chronic condition - ranging from 0.4 fewer years with the first condition to 2.6 fewer years with the sixth condition. These results are consistent by sex and race. We observe differences in life expectancy by selected conditions at 67, but these differences diminish with age and increasing numbers of comorbid conditions.