My 7yo ds (Oct bday) is about 47 inches and 48 pounds. Maybe when he hits 8? This is the same kid who doesn't have his 6y molars and does still have his top 2 front baby teeth. He rides in a belt-positioning booster or the car's built in booster (which he was not comfortable in until he was about 45 pounds, though the manual says 40 pounds).
My 4yo ds (Nov bday) is about 30 pounds (fully dressed) and 38 inches. He's going to outgrow his carseat (height wise) in the next couple of months, so I have the joy of trying to find one that can fit in the middle of our backseat so I can have a 7yo on each side of him for playdates and carpools AND that he's not too tall for.