Best Practice
Children under 35 lbs should be rear facing at all times, regardless of age. Children under two years old and 35 lbs should be rear facing at all times. 40 - 45 pounds is the current maximum rear facing limit available in the United States. It is highly recommended to rear face your child until at least age two AND 35 to 40 lbs. If you can make it to three or four, so much the better!
Children under five years and forty pounds should be harnessed in an appropriate five-point harness at all times.
Children under 5 feet tall (around eleven years old) should use a belt-positioning booster seat at all times until they pass the five step test. Best practice indicates that a younger child who does pass the Five Step Test should remain in a booster seat as long as it fits until puberty to allow the hips (aka "iliac crest") to mature.
All children should ride in the back seat until they're ready to learn to drive (around fifteen years old.)
Everyone always buckles up, no matter how old they are, front and back seat. Period.