Caries and periodontitis are the primary non-communicable oral diseases among elderly individuals. The burden of the disease increases with ageing, particularly as the elderly are tending to retain more teeth due to improvement of oral health measures and increased life expectancy. Root caries represents itself as an overlapping pathology, but not necessarily a summation of the two diseases. This narrative commentary discusses the cross-boundary nature of root caries, a periodontal-cariological condition, taking into account the multi-morbidities of ageing. The evidence includes epidemiological and pathophysiological features of root caries, and specific influencing factors of ageing, such as xerostomia, polypharmacy, functional and cognitive impairment and oral ecological alterations. Active or previous history of periodontal disease poses a risk for root caries, whereas the systemic co-morbidities of ageing may also increase the susceptibility to this pathology. It is plausible that root caries is the net outcome of coexisting risk for these conditions. There exists no standardised system for risk assessment and diagnosis that takes into account the interactive effect of caries, periodontitis and the constellation of age-specific influencing factors. As restorative treatment is challenging, cost-effective prevention and diagnosis methods are needed for vulnerable elderly populations. These may include improved clinical registration methods and establishment of individualised prevention and treatment protocols.