Best Practice
Children under 30 lbs should be rear facing at all times, regardless of age. Children under two years old should be rear facing at all times regardless of weight. (Exception: Children over 35 lbs. There is no option for children over 35 lbs to continue rear facing in the United States with seats available as of 7/2008.)
Children under five years and forty lbs should be harnessed in an appropriate five-point harness at all times.
Children under 5 feet tall (around ten 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 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.