Most of us don't think about how the seatbelt is supposed to fit. But it is designed for the lap portion to ride over your hip bones, and the shoulder portion to cross your chest and rest between your shoulder and neck.
The oldest age that states require a child to use a booster is 8. There are very few 8 year olds that actually fit the seatbelt properly. A child needs to use a booster seat until they can pass the 5 step test. Children usually pass all 5 steps between ages 10 and 12. Does your child pass?
1. Sit with your back flat against the vehicle seatback
2. Knees bend comfortably over the edge of the vehicle seat
3. Lap belt lays across the tops of the thighs, not on the stomach
4. The shoulder belt crosses the center of the chest, and rests between the shoulder and neck
5. You can sit this way for the entire ride
If your child doesn't fit the belt properly, use a booster. If you pass the test, make sure your belt is worn properly- coming from above the shoulder, across the middle, touching your body all the way down, the lap belt low and snug, flat on the hips/thighs. Keep it there. If you're old enough to be out of a harness, you're old enough that you are responsible for your safety in the car. ONLY YOU can keep yourself safe by wearing your belt right.
Here is a picture of a bruise on Anne's hip. This was from a properly fitting seatbelt. This is also 5 days after her crash. If the lap portion had been higher on her stomach, it would have caused internal organ damage.
This is a picture of Jeff's shoulder, also 5 days after the crash. The hospital was concerned enough to CT his shoulder.