I know people are always asking about car seat guidelines and safety standards and while we all knew about the rear facing until 2 per the American Academy of Pediatrics and the National Highway Transportation Safety Authority, it seems that there is confusion about booster seat guidelines.
Rear-facing Recommendation: Parents should keep children rear-facing until 2 years old, or until they reach the maximum height and weight for the seat as noted in the manual.
Once your kid outgrows the 5 point harness you can move to a high back booster seat.
Booster Seat Recommendation: Parents should have children should ride in a belt-positioning booster until they are at least 4 foot, 9 inches, AND 8-12 years old. This means high-back booster seats.
Best practice is waiting until the child is 80-100 pounds before going without a booster seat, you need to keep in mind seat belts are designed with adults in mind. Also keep your child out of the front seat until 13 years old!
For more information on car seat and booster seat safety go here.