I'm a peds nurse and CPST so I can't resist adding a few things.
Height and Weight are good indicators for older kids but age is important in younger kids as the spinalcolumn matures.
Yes, some laws state that 20 lbs is the only minimum to forward face an infant. Very scary. Others state 20 lbs and 1 year. Minimum. Ideally it is best to RF a baby/toddler as long as the weight limits of the seat allow. My 15 mos old is 23 lbs and RF. Will be until 32 lbs. In Sweden, they have seats that accommodate RF 5 year olds. Way safer as the cartilage in the spinal column of this age group is more elastic than the spinal column. The AAP and CHOP's carseat researchers support extended RF but many peds and even CPSTs are behind on this.
My state law wants kids under 8 in seats/boosters. There is also a 4'9" campaign. Ideally a kid should pass the 5 step test before abandoning the seat or booster. All points are based on size. http://www.carseat.org/Boosters/630.pdf
Maturity is a big factor- some toddlers weigh enough and are tall enough for a booster. (It's common for carseats to only harness to 40 lbs and be outgrown by height before then) but a booster will not work effectively if the child is not mature enough to follow directions and sit quietly and correctly in the seat. Most children under 4 are NOT ready for boosters.
Here's a chart on fatality statistics for Sweden (RF until about 5) vs Britain (RF at about 9 mos) http://ntf.se/konsument/bilder/diagram.jpg http://www.carseat.org/ http://www.car-safety.org/ http://www.chop.edu/consumer/jsp/div...c.jsp?id=77971