Booster seats are for older children who have outgrown their forward-facing seats. All children whose weight or height exceeds the forward-facing limit for their car seat should use a belt-positioning booster seat until the vehicle seat belt fits properly, typically when they have reached 4 feet 9 inches in height and are 8 through 12 years of age. Most children will not fit in most vehicle seatbelts without a booster until 10 to 11 years of age. All children younger than 13 should ride in the back seat.
Instructions that come with your car seat will tell you the height and weight limits for the seat.
As a general guideline, a child has outgrown a forward-facing seat when any of the following situations is true:
He reaches the top weight or height allowed for his seat with a harness. (These limits are listed on the seat and also included in the instruction booklet).
His shoulders are above the top harness slots.
The tops of his ears have reached the top of the seat.
Types of Booster Seats:
High-back and backless are 2 standard types of booster seats. They do not come with harness straps but are used with lap and shoulder seat belts in your vehicle, the same way an adult rides. They are designed to raise a child up so that lap and shoulder seat belts fit properly over the strongest parts of the child's body.
Most booster seats are not secured to the vehicle seat with the seat belt or lower anchor and tether but simply rest on the vehicle seat and are held in place once the seat belt is fastened over a child. However, some models of booster seats can be secured to the vehicle seat and kept in place using the lower anchors or top tether. (Currently, only a few vehicle manufacturers offer built-in booster seats.)
Installation Tips for Booster Seats:
When using a booster seat, always read the vehicle owner's manual and the car seat manual before installing the seat. Booster seats often have a plastic clip or guide to correctly position vehicle lap and shoulder belts. See the booster seat instruction booklet for directions on how to use the clip or guide.
Booster seats must be used with a lap and shoulder belt. When using a booster seat, make sure:
If your booster seat has lower anchors or top-tether attachments, check its booklet for installation instructions.
Common Questions about Booster Seats:
What if my car has only lap belts in the back seat?
Lap belts work fine with rear-facing–only, convertible, and forward-facing seats but can never be used with a booster seat. If your car has only lap belts, use a forward-facing seat that has a harness and higher weight limits. You could also:
Check to see if shoulder belts can be installed in your vehicle.
Use a travel vest (some can be used with lap belts).
Consider buying another car with lap and shoulder belts in the back seat.
What is the difference between high-back and backless boosters?
Both types of boosters are designed to raise your child so seat belts fit properly, and both will reduce your child's risk of injury in a crash. High-back boosters should be used in vehicles without head rests or with low seat backs. Many seats that look like high-back boosters are actually combination seats. They come with harnesses that can be used for smaller children and, later, removed for older children. Backless boosters are usually less expensive and are easier to move from one vehicle to another. Backless boosters can be used safely in vehicles with headrests and high seat backs.
Additional Information from HealthyChildren.org: