There really is not a specific height restriction. The top harness slots on the Regent are 21 inches. That is really what is most important. If you measure your child's torso height (from bum to shoulders) you will see how much room your child has. The seat is outgrown when your child hits the weight limit, shoulders go over the top of the top harness slots or the tips of their ears are above the top of the seat. When the seat is outgrown is really dependent on where your child holds their height.