Processing speed moderates the relationship between age and crystallized intelligence and influences the indirect effect of age on verbal working memory in middle-aged and older adults

This study examined the extent to which processing speed moderates the relationship between age and crystallized intelligence and influences the indirect effect of age on verbal working memory in 233 healthy middle and older adults aged 51-94 years. Participants were screened for cognitive impairment and depressive symptoms before completing a battery of cognitive tests. Data were analyzed using moderated mediation models with bootstrapping. Correlational analyses revealed significant negative associations between age and all cognitive variables. Processing speed showed the strongest negative correlation with age, while crystallized intelligence and working memory also declined with advancing age. Strong positive correlations were observed among the three cognitive indices. In the first moderated mediation analysis, processing speed moderated the relationship between age and crystallized intelligence. Specifically, age was associated with higher crystallized intelligence only among participants with high processing speed, which in turn predicted better working memory. No such effect was observed for individuals with average or low processing speed. In the second analysis, crystallized intelligence did not moderate the link between age and processing speed, suggesting differential effects. These findings indicate that preserved processing speed enables older adults to maintain crystallized knowledge, which supports working memory. Conversely, declines in processing speed may limit the protective role of crystallized abilities. Although constrained by its cross-sectional design, the study underscores the interactive roles of fluid and crystallized abilities in cognitive aging and identifies potential targets for cognitive training interventions.