I know a couple who had there son diagnosed at eighteen months.That's the youngest with the group of people that I know of. Many of the couples I know had there children diagnosed at 3 to 4 years old.
Our son was diagnosed at 6 years old. He was originally diagnosed at 4 with ADHD with the school system, but we had him seen by a physchiatrist after the pediatrician we were seeing thought his problems were more than just ADHD. That's when he was tested and diagnosed with Asbergers.