According to Arizona law (ARS §28-909), if the vehicle was made after 1972 and has belts, then all front seat passengers must wear them. All passengers 16 or under must also wear them, regardless of where they sit. Persons older than 16 in the back seat may opt not to wear seatbelts - but statistics show that in an accident, serious injuries often result when passengers are ejected because they aren't wearing a seat belt.

Of course, all children under 5 must be in a properly installed safety seat (ARS §28-907).