Background: There is a lack of research investigating whether there are differences in the domains of awareness according to the age at onset of dementia. Objective: This study is designed to investigate differences in awareness of cognitive functioning and health condition, functional activity impairments, emotional state, and social functioning and relationships among people with young onset (YOD) and late onset dementia (LOD); and examine associations between awareness and its domains with cognition, functionality, neuropsychiatric symptoms, social and emotional functioning, and quality of life (QoL) in both groups. Methods: A group of 136 people with dementia and their respective caregivers (YOD = 50 and LOD = 86) were consecutively selected. We assessed awareness of disease, dementia severity, cognition, functionality, neuropsychiatric symptoms, social and emotional functioning, and QoL. Results: People with YOD had more neuropsychiatric symptoms than people with LOD. People with YOD were more aware of disease (total score), of their cognitive functioning and health condition and of their functional activity impairments, even if this group was more severely cognitive impaired and had a worse level of functionality than LOD group. Multivariate linear regressions showed that functionality has a wide relationship to awareness for people with YOD. While neuropsychiatric symptoms and QoL has a greater relation to awareness for people with LOD. Conclusion: Different clinical variables are associated to different domains in YOD and LOD groups, reinforcing the heterogeneity of awareness in dementia.