Legally, there are many different answers to this question. State laws vary widely. Here in Ohio, our law says that children MUST use a seat until they are 8 years old or 57" tall. Unfortunately, laws do not always follow best practice of what will keep your child the safest in an accident. Seat belts are designed to fit fully grown adults, not children. The purpose of a booster seat is to make the adult seat belt fit the child's body and position the belt across the strongest bones. Here is my 6.75 year old in her Graco Affix in my 2006 VW Beetle. She is 47" tall and 46 lbs.
She has the same great lap belt fit in this mode but the shoulder belt lies too far on the edge of her shoulder. With this belt fit, she is at a greater risk of "roll out" in a crash, where the top half of her body could rotate and roll over the belt, allowing her to bend too far forward. Another common problem with no back boosters is when children fall asleep in them and are unable to maintain an upright position. The high back can help keep kids upright and in position.
There is a third option for booster seats. Combination seats start out with a harness and then, when they have been outgrown in harness mode, you take the harness out and use them as a booster. This is the Britax Frontier 85 in booster mode in our 2009 VW Jetta.
Combination seats can save you money in the long run because you can avoid buying a harnessed seat and then a dedicated booster later. They work just the same as a high back booster (some also become no back boosters as well).
Now that you know what an ideal belt fit looks like, I'd like to show you what a bad belt fit looks like.
This is Makayla, back in my Beetle with no booster seat. You can see how high the belt is on her abdomen. Children do not have the same bone density that adults do and their hips have not fully formed until puberty. When a belt is high on the abdomen like this, the belt will rip through the abdomen, crushing all of the organs against the spinal column. This can be fatal. You can also see that the belt is high up against her neck. This positioning makes it less likely that she will keep it in front of her body because it is irritating. Children tend to move irritating belts under their arm or behind their backs which can lead to a fatal injury known as seat belt syndrome when a body bisects in half in a crash because the upper body is not restrained. This is why lap only belts are not safe for anyone to ride in.
To ride safely without a booster a child needs to pass the "5 Step Test":
- Lap belt fits low on the hips, not the abdomen
- Shoulder belt lays flat on the shoulder
- Butt all the way back and back flat against the seat back
- Knees bent at or beyond the edge of the seat, feet flat on the floor
- Child is mature enough to stay in position the entire trip, even when sleeping
Take a minute today and check your child's belt fit. If you have any questions please leave a comment or send me a message!