Weight is more important than age x
But age is very important. A 6month old physically is not as able as a 9month old or a 1year old. Babies heads are too big for their bodies, they need to get into proportion before they would be able to support their own head in an accident.
So what do you do when you have a 6/7 month old that has outgrown a group 0 carseat? you can't keep them in it because they haven't reached the age limit if 9 months, you have to upgrade. Preferable you would upgrade to a group 1 seat the RF's though, but it is parental choice
And equally just because a baby hits 9mo, if they haven't reached the weight limit and still fit in the group 0 then keep them in it as long as you can.