Autism isn't a mental illness either - and never has been, although it also is in DSM-IV.
It is a condition, not an illness. It has no "onset" - it's how a person is born.
Down's Syndrome is in DSM as well - and nobody calls that a mental illness.
There are essentially two major classes of psychological challenges...
1) mental illness, and
2) developmental conditions
The first includes things like bi-polar, sociopathy, depression, Generalized Anxiety Disorder (GAD), etc. In theory, there are treatments for these illnesses - not that they work for everyone, but even on the medical side not all treatments work for illnesses. Sometimes some of these can be cured (depression), other types can be managed.
The second includes things like Autism Spectrum Disorders (ASD) (including Pervasive Developmental Disorder (PDD) and Aspie), Down's, and other developmental challenges. There is no on-set, and no medical treatment of the actual condition (there is treatment for related issues... including some of the MI that tend to go with Autism Spectrum Disorders (ASD)). The rest is accommodations and interventions and alternative approaches to teaching and to parenting. None of which would apply to MI.
You can have BOTH, but developmental conditions are not illnesses.