I think Autism has always been there but may people were not coming forward for help either due to having no insight or worried about stigma. Like Catherine said may people have been struggling in silent and diet also could be a contributing factor. We have seen a rise in young children drinking energy drinks, if they are already active this would make them even hyper. I do not have anyone with autism in my family but I know a friend who has child recently diagnosed with autism.
I do think of people like my Grandad. A man seen as clever but very quirky and odd. Traits we can recognise now as autistic but just taken at face value and dismissed back then if somebody was high functioning!