Age is a major risk factor for cerebral vascular disease. Maintaining a high cardiorespiratory fitness has been proposed as a means of improving cerebral vascular health. Age and low cardiorespiratory fitness have been associated with reduced cerebral blood flow, however their influence on the cerebral vasodilatory response to carbon dioxide (CVRCO2) is debated. Using Doppler ultrasound of the extracranial arteries, we sought to test the hypothesis that global cerebral blood flow and CVRCO2 are higher in young and old individuals who are aerobically conditioned, compared to their unconditioned counterparts. Eleven young aerobically conditioned (22±1 yr, VO2max: 65.3±0.9 mL·kg-1·min-1), ten young unconditioned (25±2 yr, VO2max: 51.1±1.3 mL·kg-1·min-1), eight older aerobically conditioned (65±1 yr, VO2max: 40.9±3.0 mL·kg-1·min-1) and nine older unconditioned (67±2 yr, VO2max: 30.4±1.2 mL·kg-1·min-1) individuals were recruited. Internal carotid (ICA) and vertebral (VA) artery blood flow were determined using Doppler ultrasound, along with middle cerebral artery mean flow velocity (MCA Vmean; transcranial Doppler), mean arterial pressure (MAP; Finometer) and the partial pressure of end tidal carbon dioxide (PETCO2; capnography) throughout. CVRCO2 was determined using a hypercapnic test protocol whereby carbon dioxide was added to the inspired air to achieve a PETCO2 of +1.5 and +6.5 mmHg above rest, while ICA blood flow and MCA Vmean were recorded. Resting bilateral ICA and global cerebral blood flow were higher in aerobically conditioned individuals (≈33% and ≈23% respectively, P<0.05 vs. unconditioned), with no significant effect of age noted. In contrast, MCA Vmean was reduced with age (60.7±2.2 cm·s-1 young vs. 42.2±2.4 cm·s-1 old, P<0.05) with no significant effect of aerobic conditioning observed. All groups exhibited similar increases in PETCO2 (≈ 4 mmHg) and MAP (≈ 4 mmHg) during the hypercapnic test protocol. ICA and MCA CVRCO2 were not different in the young and old, aerobically conditioned and unconditioned groups. These findings suggest that aerobic conditioning enhances global cerebral and bilateral ICA blood flow in both young and older individuals, but neither age nor aerobic conditioning impact upon cerebral vasodilatory capacity.