Scripturally speaking should an adult who is:
- mentally impaired,
- mentally retarded,
- unable to articulate or demonstrate
an understanding of who God is, what baptism is, or what it accomplishes be baptized?
A person is held accountable for their sins when they are old enough to have a knowledge of good and evil. This is why the Israelite children were not held responsible for their parent's rebellion (Deuteronomy 1:39).
If a mentally impaired person indicates that he wants to be baptized, then there is no harm in doing so. If he is innocent in regards to sin, all that happened is that he got wet.
But if it is the family of the person who want him to be baptized, then they need to be further taught about the innocence of children and the mentally disabled. Further, since baptism is an individual's choice, if a person cannot indicate such a choice baptizing someone without their consent will not save them.