Here's an article I found on child car seat guidelines, and it would seem they've changed again recently: http://shine.yahoo.com/parenting/aaps-new-car-seat-guidelines-change-rear-facing-booster-rules-2466904.html.
The new recommendation from the American Association of Pediatrics is that children sit in a rear-facing car seat until 2 years old or the maximum size for the seat.
They recommend a child sit in a booster seat all the way until they are at least 4 foot 9 inches tall, sometime between the ages of 8 and 12. I couldn't imagine sitting in a booster seat in middle school, and if anyone ever found out I'd have been a laughingstock. Not that I wasn't, but a booster seat adds fuel to the fire.
Kids should not sit in the front seat at all until they're 13 years old. I remember what a special treat it was to sit in the front seat when I was a kid, although I guess they didn't have air bags back them so it wasn't as dangerous. But we only had lap belts then, so at least things are improving.