The study also reiterates that children under the age of 13 should sit in the back seats rather than the front passenger side because of the danger of airbags, and that most children will need a booster seat until they are 4 feet, 9 inches tall and between 8-12 years old to ensure that the seat belt is positioned appropriately.
The recommendations are based less on age and "milestones" of birthdays (as in turning a child's carseat around on the first birthday) and more on weight and height of the child.
My children are generally petite, and my middle daughter, though normal, is extremely small, weighing in at about 40-41 pounds at 8 years old. She's a bit too large for a 5-point car seat but she is always in a high-back booster seat in any vehicle. Not being placed in an appropriate seat would likely be fatal for her in a serious car accident. A booster seat without a back causes the seat belt to ride too high on her chest and neck.
My four year old at 36 pounds rides in a high-back booster seat occasionally but we prefer her to ride most times with a car seat with a 5-point harness, especially on highways or interstates. Sometimes it's hard to get her in her car seat because she so wants to be a big girl like her sisters, but she's caught on that there's really no room for negotiation on riding in the car safely.