Occlusion is a measurement technique for assessing the visual demand and interruptability of in-vehicle information system (IVIS) tasks using a means for intermittent viewing of the IVIS displays/controls. Testing the visual demand of IVIS tasks using occlusion is gaining international support, largely due to it being a relatively simple method to use. Although an ISO occlusion standard has been published, some important issues remain; these include developing a robust and fully detailed measurement protocol. The age of participants is an important individual difference potentially affecting the validity and reliability of results from occlusion. To develop an effective protocol, the age ranges of participants need to be specified; otherwise experimenters undertaking measurements with varying age samples for the same systems/tasks may come to different conclusions. To assess the impact of age on occlusion task performance, 60 participants in various age ranges were tested on four different IVIS tasks. The results showed age-related differences; in particular, the older participants showed a greater spread of measurements. Excluding the issues of representative samples, the results imply that purely to obtain minimal inter-subject variability an occlusion measurement should use younger/middle-aged participants only.