American Academy of Pediatrics 2011 Car Seat Recommendations - 5 Steps to Safety
Infant-only or convertible car safety seat used rear-facing:
All infants and toddlers should ride in a rear-facing car safety seat until they are two years of age or until they reach the highest weight or height allowed by the manufacturer of their car safety seat.
Convertible or combination car safety seat used forward-facing:
All children two years or older, or those younger than two, who have outgrown the rear-facing weight or height limit for their car safety seat, should use a forward-facing car safety seat with a harness for as long as possible.
Belt-positioning booster seat:
All children whose weight or height is above the forward-facing limit for their car safety seat should use a belt-positioning booster seat until the vehicle lap-and-shoulder belt fits properly, typically when they have reached 4 feet 9 inches in height and are between 8 and 12 years of age.
Lap-and-shoulder vehicle seat belt:
When children are old enough and large enough to use the vehicle seat belt alone, they should always use lap-and-shoulder belts for optimal protection.
All children younger than 13 years should be restrained in the rear seats of vehicles for optimal protection.