This is my job, so I know a fair bit about this... while the previous poster is correct on the laws of Florida, the laws of physics do not change no matter what state/province you are in.
For optimal safety...
-children should be rear facing in a 5 pt harness to the limits of their seat (I tell parents to aim for 2 at a minimum whenever possible).
-children should be in a 5pt harness until 5 - 6 years old.
-children should be in a booster until 9 - 12 years of age *and* be able to pass the 5 step test before coming out of a booster seat...
5 Step Test
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
Hope that helps.