After a child outgrows both a baby car seat and a child car seat made for toddlers, he is usually not tall enough for an adult seat belt to fit him properly. Booster seats raise a child up so that the adult seat belt is positioned properly on his body. Booster seat rules vary by area, with some places not requiring booster seat use at all and others requiring their use until various ages up to and including eight years.
Typically, a child can begin using a booster seat at the age of four. By this time, many children will come close to or exceed the weight limits on most forward-facing child car seats, which vary by model. In addition, children of this age are generally able to sit still and in the proper position for a booster seat. Most areas require booster seat use for this age, and many also include mention of the child's weight. As of March 2011, Florida is the only state in America with booster seat rules allowing children older than three years to ride in a car without a restraint other than the adult seat belt.
Some areas only require the use of booster seats while the children are under the age of five years. In the United States, these areas include the states of Arizona and South Dakota. Booster seat rules that cover children under six years old are more common, with 12 U.S. states having such laws on the books. The majority of the states in the U.S. require booster seat use for all children seven years and younger.
The booster seat rules in some areas specifically address the child's height in addition to his age. A child who is not tall enough that his knees bend at the edge of the car's seat should use a booster seat, regardless of age or weight. Typically, a child will not be tall enough to use the adult seat belt alone until he is 10 to 12 years old. Without a booster seat, a shorter child may end up with the shoulder portion of the adult seat belt crossing his neck, and the lap portion resting above his hips. An improperly positioned adult seat belt can cause serious injuries in the event of a car accident.
The safest place for a booster seat is in the back seat of the car. Booster seat rules in some areas mandate this placement. Other areas do not require a specific booster seat placement, and still others allow placement in the front passenger seat only if the corresponding airbag is deactivated.