Few studies have examined the cognitive challenges faced by seniors in website use, let alone the way designers can mitigate those challenges. The findings of this ethnographic study of an Australian online community for seniors, extends present understanding of cognition and seniors, challenging the assumptions of some previous studies. This study shows that sudden adaptation to complex changes, as opposed to complexity itself, is cognitively challenging for some, particularly older, seniors. The design literature does not distinguish cognitive functions, although there are at least two distinct types of cognition that are recognised in psycho-social theories of ageing. The findings of this study suggest that, greater design effort needs to be devoted to mechanical cognition, to do with information processing and learning, than to pragmatic cognition, which concerns social interaction and communication. In addition to the main finding about adaptation to substantial change, two other cognitive considerations affecting website access for seniors were found to be: the importance of a flat navigational structure and that of providing functional, memory aids.