<jats:title>Abstract</jats:title><jats:p>Some studies show that the protective effect of higher income on health <jats:italic>weakens</jats:italic> with old age (age-as-leveller pattern), whereas others show that it <jats:italic>strengthens</jats:italic> with old age (cumulative advantage/disadvantage pattern). Many existing studies are limited in that they use single-country and/or single-timepoint designs. To overcome these limitations and better understand how the income-health gradient evolves in older age, we used cross-national and longitudinal data of the Survey of Health, Ageing and Retirement in Europe (2004–2019, <jats:italic>N</jats:italic> = 73,407) and the China Health and Retirement Longitudinal Study (2011–2018, <jats:italic>N</jats:italic> = 10,067). We operationalised health using multimorbidity and three alternative indicators (functional disability, mobility disability, and memory). We performed Poisson growth curve modelling to capture the between-participant effects of age and the within-participant effects of aging. We obtained three consistent and robust findings for Europe (patterns were observed in most countries) and China. First, the protective effect of higher income on multimorbidity, functional disability, and mobility disability was weaker for older than for younger adults (between-participant age-as-leveller pattern). Second, only the protective effect of higher income on mobility disability weakened over the later life course (within-participant age-as-leveller pattern). Third, the protective effect of higher income on memory was stronger for older than for younger adults and strengthened over the later life course (between-participant and within-participant cumulative advantage/disadvantage pattern). Longitudinal data, growth curve modelling distinguishing the between-participant from within-participant effect, and adjustments for potential confounders based on the hypothesised causal structure enabled us to better navigate the landscape of causal inference. Findings suggest that the income-related gap in physical health but not in cognitive health narrows in old age for both Europe and China.</jats:p>