They have a specific note in the 4-8 that dismisses the 4'9" after 8 years old. It sounds to me like 9 and older but under 4'9" don't need a booster, just the seatbelt. So it sounds like AND until they reach 9, then seatbelt is legal? Note is quoted below from link in original post.
"Children age nine (9) through age twelve (12), or any child through twelve (12) years of age, measuring four feet nine inches (4'9") or more in height, must be secured in a seat belt system. It is recommended that any such child be placed in the rear seat, if available. (Note: If the child is not between age nine (9) and age twelve (12), but is four feet nine inches (4'9") or more in height, he/she must still use a seat belt system meeting federal motor vehicle safety standards.)"
To my knowledge, it's just poor wording.
It means that a 15 or 16 year old who isn't 4'9" tall must wear a seat belt, but does not have to be in a booster seat, and that older kids (teens) must be in a seat belt. All children are required to be properly restrained, which includes teens. The local CPST groups here always tell parents to keep kids in boosters till they are both at least 9 years old and 4' 9" tall, and our printed cards reflect that (statewide hand out). It is worded so terribly though that it could be interpreted to mean that a 7 year old could go without a booster seat, period...but that's not the intent. If pulled over in TN, a parent with a 4, 5, 6, 7 (etc) not appropriately restrained can be fined $50. They can even be pulled over just to find out if the kids are restrained. Obviously, if you have a 7 year old who's 4 ft 9 inches tall, it might be tough to find a booster seat & seating location that works in some cars, so maybe that's the intent of the crappy wording, to excuse those super rare cases...
Note that our law states: "or according to the child safety restraint system or vehicle manufacturer's instructions.". This is what makes it "legal" in Tennessee for my 5.5 year old to be harnessed (or 8 yr olds even), 10 year olds to be boostered, and 3 year olds to rear face. As long as the seat is being used per instructions, it's fine.
Editing b/c I misunderstood part of your question...new baby brain...sorry. YES, in Tennessee, if a child is
*at least* 9 years old AND 4'9" tall, they may legally ride without a booster, but must use a seat belt. It's not perfect, but it's a heck of a lot better than Florida, Texas, or Pennsylvania, last I checked.