How to Know if Your Child is Ready for a Booster Seat | NannyPro.com:
"When your child is beginning to grow out of his car seat, it’s time to consider graduating him to a booster seat. There are several indicators that the time has arrived to make the transition, but it’s important that you don’t rush the process simply because you’re eager to get him out of a car seat. Until he’s reached the proper size and weight, your child is far safer in a five-point harness than he is in a booster seat with the shoulder and lap belt."
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