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Age-related impairments in cerebral blood flow and cerebrovascular reactivity to carbon dioxide (CVRCO2) are established risk factors for stroke that respond favorably to aerobic training. The present study examined to what extent cerebral hemodynamics are improved when training is sustained throughout the adult lifespan.Eighty-one healthy males were prospectively assigned to 1 of 4 groups based on their age (young, ≤30 years versus old, ≥60 years) and lifetime physical activity levels (trained, ≥150 minutes recreational aerobic activity/week versus sedentary, no activity). Middle cerebral artery blood velocity (MCAv, transcranial Doppler ultrasound), mean arterial pressure (MAP, finger photoplethysmography), and end-tidal partial pressure of carbon dioxide (PETCO2, capnography) were recorded during normocapnia and 3 mins of iso-oxic hypercapnea (5% CO2). Cerebrovascular resistance/conductance indices (CVRi/CVCi) were calculated as MAP/MCAv and MCAv/MAP, respectively, and CVRCO2 as the percentage increase in MCAv from baseline per millimeter of mercury (mm Hg) increase in PETCO2. Maximal oxygen consumption (O2MAX, online respiratory gas analysis) was determined during cycling ergometry.By design, older participants were active for longer (49±5 versus 6±4 years, P<0.05). Physical activity attenuated the age-related declines in O2MAX, MCAv, CVCi, and CVRCO2 and increase in CVRi (P<0.05 versus sedentary). Linear relationships were observed between O2MAX and both MCAv and CVRCO2 (r=0.58–0.77, P<0.05).These findings highlight the importance of maintaining aerobic fitness throughout the lifespan given its capacity to improve cerebral hemodynamics in later-life.