This is usually the standard across all types of rear facing car seats unless stated otherwise by the car seat manufacturer. After a child has reached the maximum height weight for a forward facing restraint they should move up to a booster seat. Once your child outgrows these given limits a regular seat belt can be used. In the last couple years there are a lot of new contenders for both rear facing weight and height limits so we have a lot more choices that will accommodate most kids. With most rear facing car seats there is a maximum weight of 40lbs. Several car seats today have rear facing weight limits up to 50 pounds which should accommodate almost every child through age 5 unless they reach the maximum rear facing height for the seat.
Lastly a child can move on to a normal safety belt once they exceed the weight height of their booster seat. These seats are designed to be carried outside of the car so they typically feature lower weight and height limits. For a long time the radian had the highest rear facing weight limit of any usa convertible and was probably second only to the true fit for rear facing height limit as well. However this isnt the case with infant only rear facing car seats which usually have a weight limit that hovers in the low 20s mark at around 22 25lbs. You should check the manufacturers rear facing height limit to be sure your child is not too tall to safely stay rear facing to the weight limit. The safest place for a child under 12 stated by law is in the back seat.
Once your baby has reached the limit for their rear facing only seat often. Most manufacturers nowadays make rear facing restraints that are able to handle children up to 40 pounds. According to the rules a child under the age of 8 who weighs between 40 and 80 pounds and is under 4 feet and 9 inches tall must have a booster seat when in the car.