The law is usually the BARE MINIMUM, and many children need to be in a booster seat until 10-12 years old. 8 years and 4'9" (57 inches) is a good minimum to start with. Then they need to look at the 5 step test and see if their child passes it in EVERY SINGLE VEHICLE they ever ride in. ALL of them. If they can't pass it, then they need to be in a booster. My 10 year old nephew is only 50 inches and still requires a booster. ALL 5 year olds need to be in a high back booster... AT THE VERY LEAST. Some 5 year olds still need to be harnessed, because they could lack the maturity it takes to sit properly 100% of the time. If the child leans over, touches the seat belt, tries moving the seat belt, etc., then they're not sitting properly and should be in a harness. The harness will keep them in the proper position 100% of the time.
If your child isn’t using a booster, try the simple test below the next time you ride together in the car. You may find that your child is not yet ready to use a safety belt without a booster.
The 5-Step Test.
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
If you answered "no" to any of these questions, your child needs a booster seat to make both the shoulder belt and the lap belt fit right for the best crash protection. Your child will be more comfortable, too!