The age-specific prevalence rates of dementia vary widely. Studies focusing on specific age groups are needed to provide reliable estimates for healthcare providers and policy makers. We estimated the prevalence of dementia, dementia subtypes and cognitive impairment in "InveCe.Ab" (ClinicalTrials.gov, NCT01345110), a single-step multidimensional population-based study of 70-74-year olds living in Abbiategrasso (Milan, Italy). We also looked for associations with socio-demographic factors and the presence of the apolipoprotein E-ɛ4 allele. The overall dementia prevalence was 3% (95%CI: 2.1-4.1%) [Alzheimer's disease (AD): 1.2% (95%CI 0.6-1.9%); vascular dementia (VD): 1.4% (95%CI: 0.8-2.2%)]. Being single was found to be a risk factor for vascular dementia; subjects born in southern Italy were shown to be at greater risk both of overall dementia and of vascular dementia. The prevalence of cognitive impairment, with or without subjective cognitive complaints (cognitive impairment, no dementia, CIND) was 7.8% (95%CI: 6.4-9.4%). As regards the CIND subgroups, the prevalence of subjects with subjective cognitive complaints (mild cognitive impairment, MCI) was 5.0% (95%CI 3.9-6.3%), while the prevalence of those without MCI (CIND-other) was 2.8% (95%CI: 1.9-3.8). The males had a higher risk of MCI and CIND-other; the older subjects were more likely to have MCI, and those born in north-eastern Italy to have CIND-other. The prevalence of AD was higher among the apolipoprotein E-ɛ4 carriers. Our data highlight the importance of dementia and cognitive impairment in the transitional period from adulthood to old age, and reveal the presence of different associations with socio-demographic and genetic factors.