Age modifies the association between physical function decline and depressive symptoms among middle-aged and older adults: a longitudinal analysis of the China health and retirement longitudinal study

To examine how age influences the association between physical function decline and depressive symptoms among middle-aged and older adults, with particular emphasis on its moderating and threshold effects. This longitudinal study analyzed five waves (2011–2020) of data from the China Health and Retirement Longitudinal Study (CHARLS), including adults aged ≥ 45 years. Physical function decline was quantified using a composite score derived from principal component analysis of multiple physical function indicators. Depressive symptoms were assessed using the Center for Epidemiologic Studies Depression Scale (CES-D). Fixed-effects panel regression models were used to estimate the longitudinal association between physical function decline and depressive symptoms while controlling for time-invariant individual characteristics. Moderation and threshold regression analyses were further performed to evaluate the interaction and potential non-linear effects of age. Physical function decline was significantly associated with increased depressive symptoms. After adjustment for physical function decline, age had no significant association with depressive symptoms. Age was not independently associated with depression, it significantly moderated this association, with the effect of function decline attenuating with increasing age. Threshold regression identified two significant age thresholds at 57 and 77 years. The association was stronger below each threshold but remained significant above it. Sex-stratified analyses identified thresholds at 78 years for men and 55 years for women. Physical function preservation may represent an important public health strategy for reducing the burden of depression among middle-aged and older adults. Age modifies the psychological impact of physical function decline, suggesting that the effectiveness of prevention and intervention strategies may vary across different stages of aging. Earlier interventions may be particularly important for women, who appear more vulnerable to the adverse mental health effects of physical decline. These findings provide evidence to support age- and sex-specific public health strategies aimed at promoting healthy aging and mental well-being.