ARS Section 28-907 deals with infants and young children. All children must be properly restrained while in a moving vehicle. The law differentiates about the type of restraint that is needed by a child.
Children 5 years old and younger need to be restrained in an approved child seat.
Children between the ages of 5 and 7 may use a booster seat unless they are at least 4 feet 9 inches tall or taller. Those who fall into this group may be secured on a child booster seat.
Children 8 years old or taller than 4 feet 9 inches will use adult restraints. (http://www.azleg.gov/viewdocument/?docName=http://www.azleg.gov/ars/28/00907.htm )