I looked more closely at this (it's what I do) and the consensus is that it is just the opposite.
..."The 2009 World Alzheimer Report, released by Alzheimer's Disease International, a nonprofit federation of 71 national Alzheimer organizations, estimates that the global prevalence of dementia, predicted to be more than 35 million in 2010, will almost double every 20 years to 65.7 million in 2030 and 115.4 million in 2050."
Global Prevalence of Alzheimer's Disease Set to Double Every 20 Years
..."It should also be noted that all of this research, while supporting the hypothesis that the rise in autism diagnoses is not due to a true increase in the incidence but rather is due to a broadening of the definition and increased surveillance, does not rule out a small genuine increase in the true incidence. A small real increase can be hiding in the data. There is no evidence upon which we can conclude, however, that true autism rates are increasing."
The Increase in Autism Diagnoses: Two Hypotheses