In a study of all men of renown, living in the 5th and 4th century in Greece, we identified 83 whose date of birth and death have been recorded with certainty.

In a study of all men of renown, living in the 5th and 4th century in Greece, we identified 83 whose date of birth and death have been recorded with certainty. Their mean +/- SD and median lengths of life were found to be 71.3+/-13.4 and 70 years, respectively. https://pubmed.ncbi.nlm.nih.gov/18359748/ https://pubmed.ncbi.nlm.nih.gov/18359748/ Dementia starts to be noticeable in modern society after about 75. I haven't found a source showing the distribution of age at diagnosis. Also the title is highly misleading: Alzheimers is only one form of dementia. From another source: A growing number of studies indicate that the prevalence 223.267.315-322 and incidence270.320-329 of Alzheimer's and other dementias in the United States and other higher income Western countries may have declined in the past 25 years,270,277.320-328.330-333 though results are mixed. 60.261.334-335 One recent systematic review found that incidence of dementia has decreased over the last four decades while incidence of Alzheimer's dementia, specifically, has held steady