The Americian Association on Intellectual and Developmental Disabilities has published a case study of a man with Down syndrome who has reached 70 with no evidence of cognitive decline. He is participating in a 20 year longitudinal study of hundreds of people with Down syndrome, and is the oldest person studied in this detail without signs of dementia.
The study concludes:
The processes regulating aging and dementia are extraordinarily complex, and it is unlikely that a single mechanism can fully explain the spectra of change and stability that occur with successful and unsuccessful aging in individuals with Down syndrome. As more information is discovered about the genes on chromosome 21, their products, the impact of the occurrence of an extra copy of chromosome 21 on the rest of the genome, and the effects of nongenetic factors, researchers will achieve a better understanding of the underlying factors and mechanisms that contribute to all aspects of phenotypic variability associated with Down syndrome (see Jenkins & Velinov, 2001). This will lead potentially to better strategies for minimizing disability and for promoting successful aging to an even greater degree than has been seen over the last several decades.
The full text is available here.
Krinsky-McHale, S, et al, Successful Aging in a 70-Year-Old Man With Down Syndrome:A Case Study, Intellectual Developmental Disabilities, 46: 3: 215–228 June, 2008