DD1 outgrew the harness in her seat a month or two before she was 5, and we moved her to a booster. I think she was 41 or 42 pounds. In retrospect, I think she was a bit young for it, probably, but she's done fine as far as sitting correctly, and doesn't sleep in the car. She rarely rides in a car, actually-- only to church on Sunday, and to her dance class. Anyway, I'm at peace with the decision at this point. The only seats that will harness a kid her size (HEIGHT is our issue) would never fit in a 3-across situation.
At least one of my twins will HAVE to be harnessed until they go to kindy at least, at 5 1/2, because it'll be until then before I can go back to work, and I can't buy a new car until I have some money, and the car I have doesn't have three shoulder belts in the backseat. We have two Radians back there with DD's Turbobooster, one forward facing and one rear facing. If the Radian won't harness them until I can buy a new car, then we're seriously screwed. (My 5 year old can fit the Radian by height by about a quarter inch
, but the twins are shorter than she was at the same age, so maybe we'll make it longer.)
So probably at least six, or until they outgrow the Radian by height, whichever comes first. There's no way on this green earth they'll outgrow it by weight-- I have very skinny kids. DD1 is only 43 pounds, although she's already probably too tall to ride in the Radian, which supposedly harnesses to 65 or 85 pounds.