Definitely not. Ask most doctors if an MRI is warranted for an autism diagnosis and they'll laugh at you.
http://autism-center.ucsd.edu/what-causes-autism/Pages/fmri....
Parent post is talking about scanning an individual. That's less useful. The second link I provide gives some reasoning: ASD is often suspected early. Many parents recognise somethings are different at about two years of age. You can't put an awake two year old in an MRI machine because you need the scanned person to stay still.