I don't even remember at what age my boys switched to boosters. I know it was right at 40 lbs, though, at least for my younger one. My older one may have been older but lighter; he probably outgrew his seat by height instead of weight. I know I had the old Century brand combo seats for them, after the Century T-shield convertibles, which came after the garage sale infant buckets. In the 90's I couldn't find any car seats that would harness past 40 lbs, but my younger child was a monster (he's over 6 feet tall now) and he can't have been past his 4th birthday by the time I had to booster him.
And I know I didn't ERF the boys, either. I remember using the Century seat rear-facing for the younger one because he was too big for the infant seat and at least I knew enough not to turn him around before he was a year old. My older one was rear-facing a little past a year because he wasn't 20 lbs yet.
In a way, I'm thankful that I got my big mega car wreck out of the way when my oldest child was only 3 months old because that meant it happened when he was rear-facing. And I sure am glad I know better now about ERF!