The World Health Organization (WHO) aims to improve our understanding of the factors that promote healthy cognitive ageing and combat dementia. Ageing theories that consider individual ageing trajectories are of paramount importance to meet the WHO's aim. Both the revised Scaffolding Theory of Aging and Cognition (STAC-r) and Cognitive Reserve theory (CR) offer theoretical frameworks for the mechanisms of cognitive ageing and the positive influence of an engaged lifestyle. STAC-r additionally

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... nsiders adverse factors, such as depression. The two theories explain different, though partly overlapping aspects of cognitive ageing. Currently, it is unclear where the theories agree and differ and what compensation mechanism of age-related cognitive decline might be better explained by either STAC-r, CR, or by both. This review provides an essential discussion of the similarities and differences between these prominent cognitive ageing theories, their implications for intervention methods and neurodegenerative disease, and significant shortcomings that have not yet been addressed. This review will direct researchers to common insights in the field and to intervention targets and testable hypotheses for future research. Future research should investigate the potential use of STAC-r in neurodegenerative diseases and provide clarity as to what combination of factors build CR, including their relative importance and when in life they are most effective.