(a) Except as provided in subsection (b) of this Code section, all citizens of this state 18 years of age or older who are not incompetent because of mental illness or mental retardation, who have resided in the county for six months preceding the time of service, and who are the most experienced, upright, and intelligent persons are qualified and liable to serve as grand jurors unless exempted by law.

(b) The following persons are incompetent to serve as grand jurors:

(1) Any person who holds any elective office in state or local government or who has held any such office within a period of two years preceding the time of service as a grand juror; and

(2) Any person who has been convicted of a felony and who has not been pardoned or had his or her civil rights restored.