It means you can turn him when he's 22#, or wait until he's 40#. Generally you should wait as long as you can.

I have that seat and they can stay rear facing until 40LBS. In the photos at the bottom of this post my daughter is seated in a Graco 4Ever and she is currently 30lbs. The best practice (from a safety standpoint) is to keep them rear facing until they max out the upper rear facing height or weight limit of their seat. Chances are they will max out one before the other, but once one limit is maxed out the seat is no longer safe rear facing. Until a child’s vertebrae have fully matured they are at risk of internal decapitation; this is when the spinal cord is severed inside the body, resulting in either paralysis or death. The maturation and ossification of the vertebrae is something that happens with AGE, not with SIZE, which is why extended rear facing (rear facing for as long as possible rather than just until age 2) is so important. Quoting from csftl.org: “Toddlers are not just small adults; their bodies are still developing and those developmental changes make them more vulnerable than an adult. A toddler’s vertebrae are connected via cartilage rather than ossified bone. Those connections are called synchondroses, which are slowly closing over time. There are three major points of ossification, each with two synchondroses. According to a study published in the Association for the Advancement of Automotive Medicine, the first station to close is the C3, second is the axis, and third is the atlas. The results of the study show: 50% probability that the primary closure of the C3 is complete at age 2 and the secondary at age 3. 50% probability that the primary closure of the axis is complete at age 5 and the secondary at age 6. 50% probability that the primary closure of the atlas is complete at age 7 and the secondary at age 8 (Yoganandan, Pintar, Lew, Rao & Rangarajan, 2011). This image to the right shows the cervical (top), thoracic (middle), and lumbar (bottom) vertebrae of a one year old (left, each photo) and six year old (right, each photo). Note the easily visible synchondroses in each. These findings show that before age two, none of the cartilaginous spaces have completed ossification. Those pieces of cartilage have the ability to stretch up to two inches. Yet only 1/4″ stretch is enough to rupture the spinal column, resulting in paralysis or death (McCall, Fassett & Brockmeyer 2004).” What this means is that a child’s vertebrae do not finish fully ossifying until ages 8-9 years old, no matter how “big for their age” they are. In fact, a child that is large for their age is actually at a higher risk forward facing than a smaller child of the same age because their head is a larger percentage of their overall weight and will exert more force on their neck in a forward facing collision. Statistics show that riders of ANY age (even adults) are safer riding rear facing. Many parents cite concerns of their child being “too cramped” in a rear facing car seat once their feet touch the seat back in front of them and their legs are required to bend when seated. Statistically, there are more leg and hip injuries reported in children who are FORWARD FACING because their legs swing up and hit the back of the front seats in a frontal collision. In contrast, in a rear facing frontal impact all of the momentum is absorbed into the back of the car seat rather than between their legs and the seat back in front of them. My oldest is 5 and is still rear facing comfortably in a Graco Extend2Fit (50lb rear facing limit.) Photos taken 5/27/21. Hours