9 years old
My son was diagnosed when he was 9. We decided to tell him right away, because he already knew he was different than his older and younger siblings. I told him as soon as we left the doctor's.
He was actually very relieved. He felt at least he now knew why he felt different and that it was perfectly okay. He embraces the fact that he has Asperger's, and his friends that have it as well he calls his "Asperger Brothers."