Researchers have found that the distinctive characteristics of autism (lack of eye contact, smiling, and communicative babbling) show up between 6 and 12 months. The article head says it's a slow decline, but the text called it a rapid decline. Maybe it depends on whether you think the period between 6 months and a year old is a long time or a short one.
The way they found this was by comparing a bunch of kids with older sibs who had already been diagnosed as autistic with some kids who had no risk factors. They used various instruments at 6, 12, 18, 24, and 36 months, counting how many interactions of what types, so they could look back and see how the kids who did develop autism really acted when they were babies.
Previous ways of seeing when autism started to show were to ask parents when kids passed developmental milestones, but parents misremember things like dates when kids do stuff, and looking at old home movies, but parents apparently turn off the camera when the kid starts acting autistic.
This is earlier than we had thought. I'm hopeful this morning. At the rate they're learning about the brain, I wouldn't be surprised if, in the few decades either some epigenetic trigger for autism is found, that we can control, or some gene therapy is developed.
This weekend, I got the ground prepared and my tomatoes planted (instead of blogging), and it's a wonderful day.