I don't have the law in front of me to reference, but I'm pretty sure the wording was 4'9" OR 8, which would mean that your 8 year old wouldn't HAVE to have a booster. If the wording is AND like it is for the regular car seat (4 AND 40 pounds), then he would still have to have a booster since he wouldn't meet both requirements. I think they have the height requirement so that (besides safety) if your 6 year old was 5 1/2 feet tall, they wouldn't be required to ride in a booster (which makes sense, since they would be taller than many adults!).