The top of the child’s ears must be at or below the top of the car seat AND the child’s shoulders must be at or below the top shoulder strap slot.
This varies by seat, so check the owner’s manual to your car seat.
Many convertible car seats have a maximum weight limit of 40 pounds. Some go higher, to 50 or 65 or even 90 pounds.
If you have a combination seat, please read the fine print carefully! While the seat might accommodate kids from 20-100 pounds, the fine print may tell you that the 5-point harness is good from 20-40 pounds and the booster from 40-100 (for example – other seats might have different starting and ending weights).