This is very interesting topic.
Yes, with our 10 year old who has a formal PDD-NOS diagnosis, we did not have him tested until he was almost five, but I did think there was something not quite right. He wanted to be social, but didn't do it as other children did. He had language, but right about 2 1/2 years old his language acquisition slowed. It didn't stop, necessarily, but it slowed way down. He went from being a very outgoing, happy baby, to being somewhat withdrawn. I noticed that he flapped his hands when he watched Teletubbies. I think I knew it was autism, but I wasn't sure. Then, when he had major trouble with changes in routine at preschool, the teachers there advised that I get him tested. It was then that I said "You think he's autistic, don't you?" Those poor teachers -- they must hate having to tell parents to get their children tested. But I'm glad they told me.
Funny thing is, his brother, now 20, seemed very normal, and always did well in school, but did have minor social problems. The more I read about autism and aspergers, the more I realized that the older son probably would have been diagnosed with aspergers.
Both are doing well now.