Transitioning to a booster seat is based on more than just weight; height and fit are critical. The vehicle's seat belt must sit correctly across the child's shoulder and hips rather than their neck or stomach. Many experts suggest keeping children in a five-point harness car seat as long as they fit within the manufacturer's height and weight limits, which often keeps them in the car seat until they are roughly 6 years old. Always check the manual for your specific car seat before making the switch.