Few studies have investigated in detail which factors influence activities of daily living (ADL) in adults with intellectual disabilities (ID) comorbid with/without dementia conditions. The objective of the present study was to describe the relation between early onset dementia conditions and progressive loss of ADL capabilities and to examine the influence of dementia conditions and other possible factors toward ADL scores in adults with ID. This study was part of the "Healthy Aging Initiatives for Persons with an Intellectual Disability in Taiwan: A Social Ecological Approach" project. We analyzed data from 459 adults aged 45 years or older with an ID regarding their early onset symptoms of dementia and their ADL profile based on the perspective of the primary caregivers. Results show that a significant negative correlation was found between dementia score and ADL score in a Pearson's correlation test (r = -0.28, p < 0.001). The multiple linear regression model reported that factors of male gender (ߠ= 4.187, p < 0.05), marital status (ߠ= 4.79, p < 0.05), education level (primary: ߠ= 5.544, p < 0.05; junior high or more: ߠ= 8.147, p < 0.01), Down's syndrome (ߠ= -9.290, p < 0.05), severe or profound disability level (ߠ= -6.725, p < 0.05; ߠ= -15.773, p < 0.001), comorbid condition (ߠ= -4.853, p < 0.05) and dementia conditions (ߠ= -9.245, p < 0.001) were variables that were able to significantly predict the ADL score (R2 = 0.241) after controlling for age. Disability level and comorbidity can explain 10% of the ADL score variation, whereas dementia conditions can only explain 3% of the ADL score variation in the study. The present study highlights that future studies should scrutinize in detail the reasons for the low explanatory power of dementia for ADL, particularly in examining the appropriateness of the measurement scales for dementia and ADL in aging adults with ID.