The World Health Organization (WHO) aims to improve our understanding of the factors that promote healthy ageing and combat dementia. Both ageing and dementia are accompanied by cognitive changes, however, some people experience more cognitive difficulties than others. Therefore, ageing theories that consider individual ageing trajectories are of paramount importance to meet the WHO’s aim. Both the revisited Scaffolding Theory of Aging and Cognition (STAC-r) and Cognitive Reserve theory (CR) offer a theoretical framework for the mechanisms of healthy cognitive ageing and argue for the positive influence of an engaging lifestyle on cognitive ageing. STAC-r additionally considers factors, such as depression, that have adverse effects on late-life cognition. Currently, it is unclear which of the two theories best explains the compensation mechanism of age-related cognitive decline. This narrative review provides an essential discussion of the similarities and differences between these two prominent cognitive ageing theories, their implications for intervention methods and neurodegenerative disease, and significant shortcomings to both theories 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.