Progression of frailty as measured by a cumulative deficit index: A systematic review

•Older people who are frail at baseline have greater likelihood to progress in frailty and die.•Age tends to accelerate transitions to higher frailty and death.•The cumulative frailty index (FI) is a robust predictor of adverse health outcomes.•The FI is a useful tool for within cohort comparisons, but it is challenging to compare different cohorts.