All vehicle booster seats must meet basic Federal safety standards. If it fails to meet these standards, then it is illegal to use and puts your child at risk.
All children should be placed in an appropriate car seat or booster seat until they are tall enough to sit safely in the back seat of a vehicle. This generally means that they can sit upright with their knees reaching the edge of the seat and their feet on the floor. Additionally, they must be able to sit with the seatbelt across them in the correct position to avoid injury in the event of a collision.
However, different states have their own specific laws related to at what age children can be safely removed from booster seats. So, it is important to check the laws in your state and adhere to them. In addition, there are age, weight, and height restrictions on each high back car booster seat. These too need to be adhered to so that your child is safe and fully protected.
In general, it is recommended that a child stays in a rear-facing car seat until they are at least two years old and then remains in a forward facing 5-point harness car seat or high back booster with harness for as long as possible. The upper weight limit for most harness car seats is 40lbs, at which point your child should transfer to a high back belt-positioning booster seat.