On average, white children are first diagnosed with autism at 6.3 years of age. African-American children are first diagnosed at age 7.9 years. http://apha.confex.com/apha/130am/te...aper_33251.htm
Most parents I have contacted, however, will tell you that they recognized these problems in their children long before they were diagnosed. With my daughter, I was commenting that she appeared to be autistic before she was 3, but she was not diagnosed for another year and a half.
I was an elementary school teacher at one time, so I had a slight advantage in that I was already somewhat familiar with autism, but I did not teach special ed, so I was hardly an "expert" in the area. I'm getting there, though, from first hand experience.