The current pathogenic scenarios of different types of dementia are based on a number of common mechanisms of neurodegeneration, such as accumulation of abnormal proteins (within or outside cells), mitochondrial dysfunction and oxidative stress, calcium homeostasis dysregulation, early synaptic disconnection and late apoptotic cell death. Ageing itself is associated with mild cognitive deterioration, probably due to subtle multifactorial changes resulting in a global decrease of a functional brain reserve. Increased age is a risk factor for neurodegeneration and key pathological features of dementia can also be found in aged brains. One of the underexplored brain structures in ageing and dementia is the blood-brain barrier (BBB), a complex cellular gate which regulates tightly the transport of molecules into and from the central nervous system. Disruption of this barrier is now increasingly documented not only in brain vascular disease but also in ageing and neurodegenerative disorders. To date, such evidence points mainly at an association between various dementia forms and disruption of the BBB. But, in reviewing such results, and taking into account the exquisite sensitivity of neuronal function to the composition of the interstitial brain fluid (IBF), which is regulated by the BBB, we would like to propose the existence of a possible causal link between alterations of BBB and conditions associated with cognitive decline.