I have asperger's syndrome. Was diagnosed from a young age too.
People don't count autism in mental illness I don't think, although I think it should be counted as ASD has caused me a lot of mental health problems. In fact doctors have even found many similarities between autistic spectrum disorder and other mental health disorders such as schizophrenia, bipolar, anxiety and eating disorders etc. If anything it puts us at more risk of developing one.
So yeah. :S
I personally think it should be counted.