Alzheimer's disease is more common in people with Down syndrome; however, symptoms of Alzheimer's disease before age 40 are not common in people with Down syndrome. The authors of the GLOBAL Medical Care Guidelines for Adults with Down Syndrome write, "Caution is needed when diagnosing age-related, Alzheimer's-type dementia in adults with Down syndrome younger than age 40 due to its low prevalence before this age." 

More information about aging and Alzheimer's disease in people with Down syndrome can be found in the webinar recording linked here. It features presenters Brian Chicoine, MD and Katie Frank, PhD, OTR/L from the Adult Down Syndrome Center.