Objectives: There is a lack of research on differences between decision-making capacity and awareness according to age at onset of dementia. We investigated the relationship between decision-making capacity and awareness domains in people with young-(YOAD) and late-onset Alzheimer´s Disease (LOAD).

Methods: A cross-sectional study included 169 consecutively selected people with AD and their caregivers (124 people with LOAD and 45 people with YOAD). Decision-making capacity was assessed with the MacCAT-T and awareness with the ASPIDD scale.

Results: People with YOAD were more cognitively impaired, but more aware of their cognitive deficits and health condition, with moderate effect sizes. We did not find any other significant differences between the groups in the other domains of awareness. In addition, there were no significant differences in the domains of decision-making capacity between groups. All PwAD presented deficits in the domains of decision-making capacity with a greater impairment in the understanding domain (YOAD = mean 3.67, SD 1.57; LOAD = mean 3.80, SD 1.22). Understanding was the domain of MacCAT-T most significantly associated with awareness domains: ASPIDD Total (p < 0.001), awareness of cognitive deficits and health condition (p < 0.001), awareness of emotional state (p < 0.008), awareness of social functioning and relationships (p < 0.001), and awareness of impaired functional activity (p < 0.001). However, age at onset only impacted total ASPIDD (p < 0.013) and awareness of cognitive deficits and health condition (p < 0.001).

Conclusions: Better awareness involved better understanding in the YOAD group. Clinically, our findings shed light on the need to consider the differences in the domains of awareness and their relationship with other clinical aspects such as decision-making capacity according to age at onset of AD.