Cognitive function declines with age. Such function depends on γ-oscillation in the frontal cortex. Pyramidal neurons, and the parvalbumin-expressing interneurons (PV neurons) that control them, are important for the generation of γ-oscillation. The mechanism by which cognitive function declines is unclear. Perineuronal nets (PNNs) mainly surround the soma and proximal dendrites and axon segments of PV neurons in the cerebral cortex. Previous evidence indicates that PNNs inhibit neural plasticity. If this is true, an increase in the number of neurons surrounded by PNNs or in the thickness or density of the PNNs around neurons could decrease plasticity in the cortex. To determine if an aging-related change in cortical PNNs occurs, we examined the influence of aging on PV neurons and whether Wisteria floribunda agglutinin-positive PNNs differ depending on the cortical area. The results showed that the number of PV neurons/mm2 did not change in many areas of the cortex as mice aged. In contrast, the number of neurons in the sensory cortex surrounded by PNNs increased as mice aged. Thus, with age, PNN density increases in some cortical areas but not in others. In addition, the expression level of PV protein in PV neurons decreased with aging in the whole cortex. We suggest that decreased expression of PV protein impairs fast spiking in PV neurons. We propose that PNNs surround more neurons as age increases. This aging-related increase in PNNs decreases plasticity in the cerebral cortex and reduces cognitive function. The first step in investigating this proposal would be to determine if PNN density increases with age.