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You can safely switch your child to a booster seat if she's at least 4 years old and she weighs 40 pounds or more or has grown too tall for her car seat (when her shoulders are higher than the top set of harness-strap slots in the car seat's back). Whatever you do, don't move your child to a booster seat simply because she's had her fourth birthday. Car seats are the safest option, so keep using yours as long as it fits. You'll also need to check the car seat law in your state (you can use the Child Safety Laws tool at the National Safe Kids Campaign site to look up local regulations).

No matter when your child finally outgrows her car seat, it's imperative that you don't skip the booster-seat step and go straight to using seat belts alone. Although many parents do this (94 percent, according to recent reports), it's a dangerous mistake. Why? Because a booster positions your vehicle's lap and shoulder belts correctly on your child's torso. Without it, the belts will be too high, falling across her belly and neck, where they can do serious damage in a crash. Given the fact that seat belts fit some shorter adults incorrectly, it's not surprising that most kids need to use booster seats until at least age 8.

When it comes to choosing a booster seat, be sure to select one that uses your auto's lap andshoulder belts (called a belt-positioning seat), rather than one designed for lap belts only (called a shield-style seat). In a crash, belt-positioning seats are far safer. (If you have only lap belts in the rear of your car, select a booster seat with an extra "energy absorber" belt.) Backless booster seats tend to provide a better fit than the high-back kind, and are more portable when you're switching them from car to car. One exception: If the rear seat of your vehicle has a low back, opt for a high-back booster seat, which provides more head and neck protection in a collision.

From Baby Center