We all know that children 2 and under (40lbs and under) should remain rearfacing in a convertible carseat. With the straps at or below shoulder lever and the chest clip at armpit level. But, what about after 2? There are guidelines for those kids up to age 12 as well.
After your child reaches age 2 or 40lbs, they can then ride in a convertible carseat with a 5-point harness until they reach the max weight or the top of their head is within one inch of the hard shell on the seat(although some seats allow a child to ride until the top of their ears are at the shell). Most five point harness seats allow a child to ride until 40lbs for the lesser models up to 85lbs for the better models. The straps should be placed at or above shoulder level. I STRONGLY suggest that your child remain rearfacing until they max out their seat and are over age 2 and 40lbs at least, although, if they can still fit their seat at 5 so be it.
Once the child reaches the max weight and height for the harness, they should then a high backed booster using the cars shoulder belt. This can be used from 40lbs to anywhere between 80 and 100lbs and 4'9" tall. Again, I cannot stress enough that you should hold off the switch as long as possible.
Children 13 and younger should always sit in the backseat. This cuts the risk of death by 36%. And all children 12 years and younger should be secured in a safety seat for their weight and height as long as they have not yet reached 4'9" and at least 80-100lbs.
Rules and carseat options
Some State Laws