Younger onset dementia (YOD) remains relatively underexplored despite its distinct clinical, functional, and socio-economic characteristics compared to late-onset dementia. Most existing research has focused on older populations, limiting understanding of early and subtle changes in younger individuals. This pilot study investigates the utility of a multimodal cognitive–motor assessment framework for differentiating individuals with YOD from healthy controls. A total of 24 participants were recruited, including 12 adults diagnosed with YOD and 12 age-matched healthy controls. Participants completed a series of standing balance and dual-task walking assessments designed to probe cognitive–motor interaction. Multimodal data were collected using functional near-infrared spectroscopy (fNIRS) to capture cortical activation patterns alongside wearable motion sensors to quantify spatiotemporal and stability-related gait and postural parameters. Results demonstrated significant differences between groups across both motion-derived and neurophysiological measures. In particular, dual-task conditions elicited greater divergence, with individuals with YOD showing pronounced alterations in gait stability and increased cortical activation, suggesting reduced neural efficiency and greater cognitive load. These findings highlight the sensitivity of cognitive–motor paradigms, particularly under dual-task conditions, in detecting early functional impairments in YOD. This study provides preliminary evidence supporting the integration of multimodal sensing approaches for improved assessment of YOD. The findings have implications for earlier detection, better characterisation of functional changes in YOD, and the development of targeted interventions.