The American Academy of Pediatrics goes a little further and says that a child isn't ready for regular seat belts until they reach 'about 4'9" in height and is between 8 to 12 years of age.'
To make it even easier, remember that seat belts don't fit properly until the lap belt lays across your child's upper thighs (not his stomach) and the shoulder belt fits across his chest (not his neck).
If you need some convincing about how important this is, consider that in 2001, there were 44,642 injuries from motor vehicle accidents for children three years old and younger, and that grows to 76,248 injuries for children four to eight years of age. Although injuries from motor vehicle accidents aren't tracked as to whether or not a child was wearing a car seat, it is likely that this increase in injuries for older children was because they are less likely to be in a proper child safety seat than a younger child.
So whether you live in Florida, which only requires children up to age three to be in a car seat, or in Wyoming, where the limit is age eight, after your child is about 40 pounds and outgrows his car seat, be sure to graduate to a belt positioning booster seat instead of simply going to seat belts.
It can sometimes be hard to convince school age children about the importance of using a booster seat, especially if many of their friends are already just in seat belts. To help him/her get on board with using a booster it can help to be firm about his sitting in a booster seat and use it all of the time. Make using a booster seat one of the non-negotiable rules of your household and don't give in on the issue.
1) don't refer to it as a car seat or baby seat and instead use the terms booster seat, big boy/girl seat, or just safety seat. If your car's back seat has headrests, consider using a backless booster, which to many kids doesn't look like a 'real' car seat.
2) talk about the benefits of sitting in a booster, which besides safety include being able to look out the window, having cup holders, and being more comfortable with the arm rests, etc.