This paper presents the design and usability validation of a telematic test aimed at the early detection of Mild Cognitive Impairments in persons ageing between 50 and 80. The test represents a 3D domestic environment in which users perform 5 daily life activities that involve various cognitive skills: short-term and mid-term memory, attention, executive functions and spatial orientation. The test is also aimed at assessing cognitive impairments in persons already diagnosed or having neurodegenerative dementia. In order to make it usable by seniors without previous computer literacy, the test integrates different accessibility features: automatic navigation, assisted selection and feedback mechanisms. The usability of the system has been tested with a set of target users. The results show that users are able to understand the virtual environment and the tasks. In addition, they learn very quickly to manoeuvring in it. Moreover, they find it attractive.