Frailty and social isolation are independently associated with dementia risk. This study examined whether frailty predicts dementia differently for older adults living alone versus with others.

Participants were 967 community-dwelling, dementia-free older adults from the Sydney Memory and Ageing Study (2005–2020). Frailty was assessed using an adapted Fried Frailty Phenotype (robust, pre-frail, frail). Dementia was diagnosed via biennial neuropsychological testing and clinical consensus. Cox regressions analysed associations between baseline frailty, living arrangements (alone vs. with others), and incident dementia over 12 years.

Frail individuals had 1.74 times higher dementia risk than robust individuals (HR = 1.74, 95% CI: 1.16–2.60, p = .007). Living alone was not independently associated with dementia (HR = 1.03, p = .825). The interaction between frailty and living arrangements was non-significant (HR = 1.23, p = 0.600). Exploratory stratified analyses showed frailty predicted dementia among those living alone (HR = 1.92, 95% CI: 1.06–3.51, p = .033) but not those living with others (HR = 1.42, p = 0.224).

Frailty is a significant predictor of dementia risk in older adults. While the formal interaction test was non-significant, exploratory findings suggest frail individuals living alone may be particularly vulnerable, warranting further investigation and potential targeting for combined physical and social interventions, pending replication.