Just for "fun" facts..
Look at these statistic, it shows the difference in children being injured or killed in cars accidents in Sweden and Great Britain, per 100000 children
In Sweden it is recommended to sit RF up till 4-5 years of age. You can especially see it on injured/deaths of the four year olds.
You can also see the difference between Sweden and France and Sweden and Germany
HERE on page 9.
__________________
DS 5YR, 40 lbs, 43.7", RF in a Britax Two Way Elite.