Off the top of my head (or hopefully corrected if wrong) i believe you would have to have stayed 15+ days in a mental institution, or has been declared insane.
- I know it is spelled out in the firearms related RCW somewhere, but i just got home from work so I'll leave the surfing to you
As for Autism, Downs, or any other similar condition... i honestly cant say. Interesting question indeed. Might depend on the severity and if that one is self reliant or has any criminal charges etc??? - just a guess.