Vaccines and Autism: Questions

This Sunday at 8 pm ET, CNN will be running “Autism Is A World”, an Academy award nominated documentary about autism. When it comes to autism, here is one of the key questions we should be asking:

There is no such thing as a genetic epidemic. Yet U.S. autism rates have gone from about 1-in-5000 in the 1980s to 1-in-166 today. But if autism is purely genetic (without an environmental "trigger") and has always been prevalent at the same constant rate, then where are the 1-in-166 autistic 25-year-olds (those born in 1980)? Where are the 1-in-166 autistic 55-year-olds? Why can't we find them?