Aim:  Although several researchers have examined the effect of age on disease progression in patients with Alzheimer's disease (AD), the results are controversial. We investigated the effect of age on the rate of progression of cognitive impairment and on regional cerebral blood flow (rCBF) deficits using longitudinal single photon emission computed tomography (SPECT) studies. Methods:  We divided 75 patients with AD who were followed up for 24–90 months into the younger AD group (n = 32, age at initial examination ≤75 years) and the older AD group (n = 43, age at initial examination >75 years). We assessed changes in Mini‐Mental State Examination (MMSE) scores and rCBF between initial and final examinations. Results:  The change in the annual MMSE score was greater in the younger AD group than in the older AD group. The mean annual MMSE score changes correlated significantly with age at the initial examination. The initial and follow‐up SPECT studies showed more severe and extensive rCBF deficits in the younger AD group than in the older AD group. Conclusion:  Our longitudinal SPECT study showed that the rate of progression of cognitive impairment and the rCBF deficits were higher in younger patients with AD than in older patients with AD. Therefore, age is an important factor to consider, not only in the diagnosis, but also in the treatment and prognosis of patients with AD. Geriatr Gerontol Int 2013; 13: 555–562.