How old must a child be to use a booster seat?

A child typically must be at least 4 years old to use a booster seat, although specific age requirements can vary by state or country. Children should also meet the weight and height guidelines set by the booster seat manufacturer, generally weighing between 40 and 100 pounds and being at least 40 inches tall. It's important that they can sit properly in the booster seat, with the seat belt fitting correctly across their shoulders and lap. Always refer to local laws and guidelines for the most accurate information.