It is important to note that all front seat passengers and people under 16 years of age are required to wear seat belts, both the lap and shoulder belt (with the exception of infants and young children) in a moving vehicle. This statute is a secondary enforcement law, which means you must be stopped for another primary violation. If you are not properly restrained you can be issued a citation.
ARES 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)