A child should not use a booster seat unless he/she is BOTH:

• at least 3-4 years old AND
• at least 40 pounds

Children 3 or 4 years old and 30 to 40 pounds could use a combination seat, which is a seat that starts off as a 5-point harness and then turns into a booster seat once the child is older, heavier, taller, and more mature.

Why shouldn't my 3 year old 30 pound child use a booster?
The 5-point harness of a forward-facing car seat provides the best protection for toddlers, because it not only restricts movement, ensuring that toddlers are in the proper position should a crash occur, but also distributes the crash forces over a larger area of the body when compared to a safety belt and booster seat (two chest straps with the safety seat versus one chest strap with the safety belt.).   While many booster seats say that they can be used for children weighing as little as 30 pounds, it is best to wait until the child is at least 40 pounds before using a booster seat.

Do I HAVE to put my child in a booster once he/she is over 40 pounds?
No, many 40 pound children are not mature enough to use a booster seat.  If your over 40-pound child leans over to fight with a sibling, picks up dropped toys, hangs out the window, unbuckles his seat belt, etc., then he is NOT READY for a booster seat.   In this case, you should look for a child safety seat that has a 5-point harness and can accommodate a child weighing more than 40 pounds.  Some of these seats will turn into boosters later on, once the child is older, heavier, taller, or more mature.