What are the requirements for using the Maestro Sport as a belt-positioning booster seat?

The child must weigh between 18 to 49.8 kg (40 to 110 lbs), measure 112 to 145 cm (44 to 57 in.) in height, and be at least 4 years old. The tops of the child's ears must also be at or below the top of the child restraint headrest. Once the child exceeds any of these requirements, you must discontinue use of the seat entirely. (Page 4–5)