Previous guidelines from the AAP have always made this recommendation, but also cited one year, or 20 pounds, as the minimum requirements for flipping the seat around. This minimum requirement is no longer recommended. A 2007 study in the journal Injury Prevention showed that children under age 2 are 75 percent less likely to die or be severely injured in a crash if they are riding rear-facing.
The reasoning is that a rear-facing seat does a better job of protecting your toddler's head, neck, and spine in the event of a crash, since it distributes the force of the impact through the entire body.
Once your child has outgrown her rear-facing seat, she should transition to a forward-facing seat with harness until the height and weight recommendations are again exceeded.
After she grows out of that, she should ride in a high-back, belt-positioning booster until the lap-shoulder belt fits her. Booster seats help position adult seat belts correctly on children's smaller frames, and are recommended until your child is at least 4 foot nine inches tall and 8-12 years old.
Finally, no children should ride in the front seat until they are at least 13 years old.
For more information, read the AAP's policy statement on their updated car seat recommendations.
Read more about car seat and booster seat safety for kids.