A child must be 4 years old and have reached the maximum height allowed in a forward-facing car seat before using a backless booster seat, as of 2015. Parents who own a forward-facing car seat should check the manufacturer's instructions for the maximum height it permits.
Manufacturers often create child safety seats to serve multiple age, height, weight and safety ranges. Some forward-facing seats have extended weight or height safety ranges that allow them to act as a booster seat if the child does not meet all the requirements for a backless booster seat. When choosing a child safety seat, parents should review the government safety recommendations as well as the operating ranges for each seat.