I have had several parents ask me if they should tell their child that they have autism, and if so, when should they tell their child. I believe that telling a child they have autism can be a very empowering experience, especially if you explain what autism is in a thorough fashion, how autism affects how they perceive the world, and to also point out their strengths as it relates to autism.
Learning that I had autism was one of the most important events of my life. It was like a blind person learning they were blind or a deaf person learning they were deaf. I know knew why I was the way I was. Actually I did not know much about myself before, and now I understood I was different.
I recently read I Want to Be Like You by Travis Breeding. He wasn't diagnosed until his twenties. Also, John Elder Robison author of Look Me in the Eye wasn't diagnosed until much later in his life, his forties, I think. Back then, Asperger's wasn't recognized in the US. Both of these people became empowered when they learned about having a form of autism. They now understood themselves and saw what aspects of themselves were different from everyone else. And, with that knowledge they were able to work on those aspects.
Having an autism diagnosis is like having a map of who you are. I know of some people that desire for other people to "get them." With this diagnosis, I don't have to worry about someone that gets me because I have the map. I know what aspects of myself are a result of my autism, and because of that I know what people without autism are typically like. Also, I am able to identify these autistic tendencies, and if I feel like they are stopping me from living my life to the fullest, I often times can figure out how to mute a certain symptom.
Most people who receive the diagnosis later in life felt lost during their middle school and high school years. Some, even contemplated and even attempted suicide. I believe that if they had knowledge about being autistic, that life could have been much easier for them in their school days.
So, when do I believe is a good time to tell your child they have autism. I think when they have the cognitive ability is the best time to understand. I think the 5th grade is a good start or some time in middle school. I think high school is too late. A lot of social things start to build up in middle school, and in high school everything goes to the next level socially.
So, empower your child. Just be sure that you explain in a fashion that is positive for that person. Focus on the strong attributes but don't gloss over the perceived negative aspects of autism as it relates to your child.
I wish you and your child the best, and hope both of you have a rewarding future. :)