I think your question is a good one, and the posts here are downright funny.
I’d like to know the answer to your question.
And I think it’s important to distinguish mentally retarded. My daughter is mentally intellectually disabled with an IQ of 70, and she can vote. She also hold two jobs and is working on her associates degree in photography.
A guide to state laws on voting for people with mental disabilities is at
http://www.ndrn.org/issues/voting/resources/state_voting_rights_MD_laws%5B062304%5D.pdf . In general, I think people lose the right to vote only when they have been declared “insane” or “incompetent” by the state.