Booster seats are designed to raise a child up so that the lap and shoulder seat belts fit properly over strong growing bones. Benjamin Hoffman, MD, FAAP, a pediatrician and certified car seat technician, guides parents on how to properly use and install booster seats in your vehicle. Booster seats should be used until your child can correctly fit in the adult lap and shoulder seat belts, typically when they are around 4 feet 9 inches in height and 8 to 12 years old.