this is the actual law:
(1) Infant-rear facing infant or convertible seat, birth to at least 1 year and 20 lbs(2) Toddler-forward facing car seat or convertible seat, at least 1 year and 20 lbs to around 4 years and 40 lbs(3) Booster-usually from around 4-8 years and 40-80 pounds(Height and weight limits vary for different seat manufacturers, so always check the seat instructions and vehicle owners manual)(4) Seat Belt-4'9" tall and around 70-80 lbs
But they just changed recommendations to rear facing to 2 year old and 25 pounds.
If you are able... leave them RF as long as possible (as long as the seat allows). There are numerous studies that show that is safest for them to be RF until they have way stronger neck muscles because their head weighs so much compared to the rest of their body. http://www.cpsafety.com/articles/stayrearfacing.aspx Here is some info on why it's better. I have an 18 mo old/ 25lb DS and he is still content to ride RF. Since we never switched him, he doesn't know that he can be the other way. He doesn't always love being in his carseat, but I don't want him turned around yet.
The AAP advises
parents to keep their toddlers in rear-facing car seats until age 2, or
until they reach the maximum height and weight for their seat.
DS has always been a big baby/toddler. His rear facing weight maximum was 35 pounds (which he hit around his 2nd birthday).
I also encourage you to check this out: