20lbs and 1 year is the law in most US states, it's not official guidance here. Law here is they must be 9kg, nothing on age. If they're on the 99th centile and hit 9kg at 4 months, it's legal :(
Official guidance is to keep them in the infant seat until they grow out of it but the only place I've ever seen this in an "official" capacity is on the RoSPA website about child seats, which most parents don't access. There were a mixture of old and new car safety leaflets in my children's centre, the old ones advising parents to turn their children FF when they could sit unsupported, I think the new ones were only printed when the law changed in 2003, so god knows why they still had even older ones than that, I guess nobody reads them. My health visitor told me when DS had his 9 month check that as he was over 9kg, it was unsafe to have him rear facing. I had to insist I knew he was still fine in his current seat, she looked dubious and then told me "Oh, you probably know what you're talking about since you had that car accident."
Most people I know seemed to turn them around at 9 months or so because that is the category the seats are sold for, despite the first seats being advertised for up to 12-15 months, and DS fitted in his until 18 months and he is 50th centile for height so pretty average.