Age-period-cohort decomposition and projections of the frailty index among older adults in China

Introduction: The frailty index is a composite indicator of older adults' health status. This study examines temporal trends in frailty among China's older population and seeks to disentangle the respective roles of aging, cohort replacement, and period-specific environments. Methods: Using longitudinal data from the Chinese Longitudinal Healthy Longevity Survey (CLHLS) and drawing on the accelerated longitudinal design framework, we decompose changes in frailty along the age-period-cohort (APC) dimensions. We estimate the marginal contributions of age, survey period, and birth cohort to frailty patterns among older Chinese adults and, on this basis, generate projections of future frailty trends. Results: First, the frailty index increases strictly monotonically with age. The cohort effect exhibits an overall declining trend, such that later-born cohorts show lower frailty at the same age and in the same period. The period effect shifts upward over the sample window, suggesting that the influence of macro-level environments and institutional change on frailty is not a simple unidirectional health gain. Second, frailty displays pronounced gender and urban-rural disparities: rural men have the lowest overall levels, whereas urban women rise more rapidly at advanced ages and remain the most frail. Moreover, the urban-rural gap among women widens faster than that among men, and within-city gender disparities also intensify more rapidly than within rural areas; advanced old age emerges as a critical interval in which urban-rural and gender inequalities interact and amplify. Third, frailty among China's future older population is projected to worsen continuously, albeit at different rates across groups. The largest increase is expected among urban older women, followed by rural older women, while frailty among rural older men is projected to surpass that of urban older men after 2035. Across years, the upward shift is generally larger for rural groups than for urban groups and is more pronounced at advanced ages, implying that the future burden of frailty may be increasingly concentrated among rural older adults. Discussion: These findings highlight the importance of an APC-informed and ALD-based perspective for interpreting frailty dynamics and for producing structurally grounded projections. The projected widening of rural disadvantage at advanced ages underscores the need for forward-looking public health and long-term care planning with attention to urban-rural and gender inequalities.