One of the most common questions asked of our team of Child Passenger Safety Technicians is if their child is ready to switch to a booster seat. Rather than switch your child when they meet the minimum to sit in a booster (roughly 3 years old AND 30lbs), we prefer for children to make the leap once they have outgrown their 5 point harness, front-facing convertible seat.
At this age, with your little one outgrowing his or her clothes and shoes so quickly, you might naturally think you need to move up to a booster car seat, but your child is safest in a 5 point harness as long as they are within the seat's height and weight limits AND your child's shoulders are not higher than the top harness slots. (For example, a child might be within the height limit of their 5 point car seat, but if they have an unusually long torso and their shoulders are above the top harness slots, it's time to change seats.)
Some parents are eager to switch seats because booster seats are often more portable and easier to install/use or because their children complain about having to sit in a "baby seat", but a 5 point harness does a better job of distributing crash forces across a greater portion of your child's small body.
So when is it ok to switch to a booster? We prefer to wait until 1) a child has outgrown their 5 point harness 2) he/she is AT LEAST 4 years old and 3) weighs AT LEAST 40lbs. They also have to able to sit properly for the duration of the car ride, meaning, not taking the shoulder belt off, sitting upright and not leaning or slumping out of their space.