THe big stressor here to me is height and weight and "fit". I don't even like the age thing attached to it -- why have it? Just make it size/mass dependent -- kids come in all shapes and sizes. Now in baby seats, I can understand it, because a big big baby may be large enough to "face forward" yet not have the neck muscle control such to avoid injury if facing forwards. But the difference in muscle or cognitive development between 20 kids aged 7 to 9 is surely much less the safety issue than their size/mass and the "fit" of the seatbelt would be.
We have one son who has always been tall and at the 100%+ for his age group, whereas son #2 is built on a much smaller frame. Son #1 had to drop use of the child seat 3 yrs ago at age 7-- he was too tall for boosters and of reasonable mass. Son#2, however, is now 8 1/2 and he still, to me, needs a booster depending upon the vehicle. For example, in our van, he can't ride in the middle-row captain's chairs without a booster, but his fit is quite good in the back row or in my dh's sedan. You have to examine the seat from all angles. Does the seat belt hit your child mid-chest, or across his/her neck? Is the lap belt portion properly positioned around the hips?Does your child's knee bend at the seat edge, or are your child's legs sticking out like Lily TOmlin in the big rocker? Is the seat belt sloppy or comfortably snug? I look at all those factors in deciding when a child can safely sit without a booster in a given seat --
I have always been amazed by how many parents no longer use child seats of any sort once their child grows out of the "bucket" (baby seat). Friends of our kids have all learned that if you wanna ride in our car, you'll ride in a safety seat if your height/weight/fit so mandate. I even bring extra safety seats for field trips. Of course, some of the kids balk and say, "But I don't HAVE to in MY car!" And I always respond chipperly, "Well then, my good man (or lady) you are in for a real TREAT! Just THINK -- No more seatbelt in the neck, no straining to see out the windows, you'll be riding tall like the teenagers do -- you are gonna LOVE it! Let's try it out now and see how you fit! Ooooo -- you look at LEAST 5 yrs older! Are you in the First grade? Or is it maybe the Fifth? Good gravy, now I can't tell! And you're so SAFE too ... gotta love that!" I act a bit goofy with it, (par for the course with me) but my own kids (who would otherwise be embarassed) are grateful to have the playing field leveled, so to speak, and on subsequent field trips, I've always got plenty of kids clamouring to ride with me, so it must work. I try to build up a car seat like it's the best thing since bubble gum... After all, as the Michelin ad used to say, "She's not your daughter... but if she's riding in your car, she may as well be." Any passenger in my car is gonna gonna ride as safely as I can possibly provide, EOS.
Anyhooo, stepping down from Soapbox. Thanks for the heads-up on the booster issue in NC. I will pass it along to folks here that I know sojourn annually to Myrtle Beach. Maybe they will buy boosters (and then they can help me drive on field trips...)
So, be careful driving through PA and NJ also! 
