Neurodegenerative diseases such as Alzheimer's, Parkinson's or Amyotrophic Lateral Sclerosis are generally of sporadic origin, but the risk of being affected increases dramatically with age. Progress in understanding brain aging contributes to define precisely the relationships between aging associated physiological and pathological processes. Aging is induced by various stresses, which turn somatic cells to a senescent state. Senescent cells cannot divide but acquire a typical phenotype and release chemicals that trigger inflammation. In the brain, glial cells such as astrocytes or microglia as well as neurons are affected. Dendritic branching declines, resulting in a decrease of synaptic plasticity and cognitive performances, associated with brain aging. Aging alters also glymphatic washing, the process by which brain catabolites are eliminated. The neurodegenerative diseases are proteinopathies resulting from pathological conformations of physiological proteins. Normally washed by the glymphatic flux, these proteins accumulate and aggregate in its absence and then trigger neuron death.