When children outgrow their car seats, they are ready to move to a booster seat for backseat riding. Children should ride in booster seats until a car seat belt fits properly, the AAP recommends. This is usually when children are about 8 to 12 years old and about 4 feet, 9 inches in height. All children younger than 13 should continue riding in the backseat.  For Diono seats, the Radian 3RXT converts to booster mode for children up to 120 pounds, but the minimum weight for booster compatibility is 50 pounds. The Rainier does not convert to a booster at all.