J.H.
The very minimum for being able to turn her is 1 year of age AND 20 pounds; so, even though she's 20 pounds now, she still needs to rear face until she's at least 1 year old. Is she in an infant bucket seat? You can buy a convertible car seat that will rear face and forward face. Britax is a great brand, and I'd go with the Marathon or the Boulevard. They are more expensive than other seats, but they last for the long haul. My daughter is 19 months and in a Boulevard, still rear facing. The limits for that seat to rear face are 35 pounds or a certain height (don't remember, don't have the car seat book in front of me.) If cost is an issue, you can get a Cosco Scenera from WalMart for cheap, and it rear faces to 33 pounds, and forward faces after that. The downside to buying a cheaper seat like that is that you will eventually need another one after it, because it only has a 40 pound weight limit total. The Boulevard has a 65 pound limit, and there are others that go up to 80 pounds.
The bottom line is that it's 5 times safer for your children to rear face until they are at least TWO years old. The law states 1 yr and 20 pounds, but the AAP is now recommending 2 years and 30 pounds. We will rear face until she reaches the 35 pound weight limit or the height limit, which will probably last us until she's 4. The legs dangling or hitting the seat cause no problems - they just learn to sit a different way (cross legs, feet up on back of seat, or dangling over the sides of the seat). In the event of an accident, it's far less likely that the child will break her legs rear facing than if she were forward facing where the chances for internal decapitation are very high. It's easier to fix a leg than it is to fix a spine or neck. I always say "legs - cast it, neck - casket."
If you have any questions or want crash test videos of rear facing vs. forward facing, let me know!