In common with other organ systems, the cerebrospinal fluid circulatory system is subject to senescence. Cerebrospinal fluid production and turnover rates decline. The choroid plexus calcifies and its blood supply falters. Microvascular disease violates the integrity of the blood-brain barrier. The arachnoid membranes thicken. Arachnoid villi occlude and degenerate. The consequent functional losses are exacerbated by the deterioration of other interacting organ systems. Eventually, the cerebrospinal fluid circulatory system may fail, resulting in stagnation, contamination, compositional deficiencies, and impaired clearance of noxious substances. The hypothesis is that senescence of the cerebrospinal fluid circulatory system plays a part in some dementias of the aged.