70 years old and going strong with Down syndrome and no dementia

In the world of Down syndrome, 'Mr. C' is a rarity. A real person whose progress has been tracked for the past 16 years, at seventy, 'Mr. C' has well surpassed the average life expectancy of a person with Down syndrome, currently in the late fifties, but in the teens when 'Mr. C' was born. Further, 'Mr. C' does not exhibit clinical symptoms of Alzheimer's disease, which is almost a given for people with typical Down syndrome over 65 yeas of age.