While it’s true that most children can be transferred from the high back booster to backless booster when they turn or are over 4 years old and weigh over 40 pounds, it’s important to remember that children grow both physically and mentally at different rates. For instance, some children, although 4 years old, may not actually meet the height requirements (around 40") of backless booster seats. Others may meet all the physical requirements, but may not be mature enough to sit still in a booster seat (as mentioned, they may try to move their seat belt in unsafe positions). The right time for a child to sit in a backless booster seat is when they have met all the requirements of that booster seat. On paper, this includes children aged 4 to 11 years, measuring 38/40 to 57 inches in height and weighing between 40 to 100/120 pounds (depends on the model).