Booster seats are like a combination of a glorified phone book and an advanced pelvis for your child. While most of us rode around without car seats or booster seats or even seat belts in the conversion van or in the back of a pick-up when we were kids (and we made it out just fine), we were the lucky ones. The kids who didn’t make it aren’t around to advocate for advancements in safety. Their parents had to do all the work in their honor, to which I would like to say thank you.
What most parents don’t understand is that the seat belt in a car is designed and tested for a manikin that is 5’10” and 180lbs. Seat belts don’t fit most adult women much less an average 8 year old child.
The ambiguous cut-off for kids to be old/big enough to not sit in a booster seat is somewhere around age 8, or 4’9”, or 80lbs. In fact, most kids don’t reach 4’9” until somewhere between 9 to 12 years old, according to the CDC (boys and girls).
While your 9 year old might fit properly in say a Mini Cooper without a booster seat, they might still need one in an SUV because of the larger seats and attachment points of the seat belts.
Proper fit has much more to do with the placement of the seat belt across the child’s body, than it does with the child’s age.