Arizona vs California

For families moving or driving between Phoenix and Southern California, the good news is that the booster rule matches: both Arizona and California keep a child in a car seat or booster until age 8 or 4 feet 9 inches (about 57 inches), and both release a child to a seat belt at that point. Where California is clearly stricter is at the youngest ages. California requires rear-facing under 2 and the back seat for any child under 8, two rules Arizona does not have. So the move rarely changes anything for a school-age child, but it can change a lot for a baby or toddler.

California requires a child under 2 to ride rear-facing unless they already weigh 40 pounds or stand 40 inches tall, and it requires children under 8 to ride in the back seat where one is available. Arizona sets no rear-facing age (its restraints must meet the federal standard, but the law does not name an age) and does not require the back seat. In practice, a parent who turns a 15-month-old forward, or seats a 6 year old up front, is fine in Arizona but not in California. Pediatricians recommend rear-facing as long as the seat allows and the back seat under 13 in both states.

Arizona requires a child at least 5 but under 8 who is not more than 4 feet 9 inches to be in a child restraint such as a booster. California requires a car seat or booster until age 8 or 4 feet 9 inches. The effect is the same: a child graduates to a seat belt at 8 or once they reach 4 feet 9 inches in either state. If your question is only about when the booster comes out, Arizona and California give the same answer.

California's fine is higher: a base of about $100 for a first offense and $250 for later ones, plus county penalty assessments, versus Arizona's flat $50 civil penalty. California is also changing in 2027: Assembly Bill 435 will require a seat belt to pass a five-step fit test, so a child who cannot pass keeps using a booster even after age 8 or 4 feet 9 inches. Arizona has no comparable rule. If your trips cross the state line, set up the car to California's tighter requirements and you will be covered everywhere along the way: child under 2 rear-facing, child under 8 in the back seat, and a booster until the belt truly fits.