In short
A frailty index is a composite measure of age-related vulnerability calculated as the proportion of a large set of potential health deficits — symptoms, diseases, and functional impairments — that a given individual has, out of the total number tracked.
The deficit-accumulation frailty index, developed by Kenneth Rockwood and Arnold Mitnitski, is built by counting how many of a predefined list of health deficits (commonly 30 to 70 or more items, spanning things like difficulty with mobility, vision problems, chronic diseases, and cognitive symptoms) a person has, then dividing by the total number assessed. A person with deficits in 14 of 40 tracked items has a frailty index of 0.35, for example.
This differs from the frailty phenotype model (Fried et al., 2001), which defines frailty as meeting three or more of five specific physical criteria: unintentional weight loss, exhaustion, weakness (measured by grip strength), slow walking speed, and low physical activity. Both models are validated and widely used in geriatric research, but they measure somewhat different (though correlated) constructs — deficit accumulation captures broader multisystem burden, while the phenotype model targets a specific physical frailty syndrome.
Across large cohort studies, frailty index scores predict mortality, hospitalization, and disability better than chronological age alone, and this has made frailty indices a standard research tool in geriatric medicine and an increasingly used input into biological-age composite scores in longevity research. Frailty indices are validated primarily in older adult populations (typically 65+) and were not designed to differentiate fine-grained aging trajectories among healthy younger or middle-aged adults, which limits their direct applicability in that group.