Because I know that over time this IIHS study, http://www.iihs.org/research/topics/boosters/default.html, will question some of what we have presented, I offer a few quick updates. The booster shown in this video is adequate, not ideal. As the IIHS study demonstrates, the farther off the shoulder the seat belt positioner is, the more likely it is to be ineffective. This is why safety advocates, like myself, encourage parents to purchase a booster, with a back that adjusts to the child height and positions the shoulder belt over the child’s shoulder in stead of on the shoulder. The IIHS study pictures illustrate to this subtle issue. More important is to make sure the design of the booster seat is complimentary with and compatible with the design of the vehicle seat. This is after making sure it is appropriate for the child’s physiology; weight and height proportionally. We do not usually address child maturity, so I will throw it out for thought and encourage you to consult with your local Car Seat Technician for assistance on all of these matters. Good day.

This video doesnt help. Most of what he said was obvious, the question asked is WHEN, and he was very cloudy on that point. He mostly explained about harnesses and seat belts. He went into automatic answer mode from the looks of it, reciting a segment of his standard spiel.  He did not clarify the question imho.

Akralia, the reason that carseatguy does not give a specific time to switch to booster is that this varries from child-to-child, vehicle-to-vehicle & situation-to-situation.  There simply is no specific time other than “when the child outgrows the harness carseat”.  Even then, if the child has special needs or simply isn’t mature enough to sit as still as a crash test dummy for the entire length of every ride, properly positioned 100% of the time (even while sleeping), then that particular child needs a High Weight Harness carseat option.