Ds2 is still in a rearward baby car seat. He is very small for his age and still seems to fit it fine. His head is not at hte top yet. I know I have read many times about when you should move them into next stage, but I can't remember. Ds1 was big for his age and definately out of his well before 1 year old.
I just looked at the manufacturers website and it says up to 13kgs for their current 0+ seat (mine is quite old as ds1 had it first). He is not 13 kgs yet as he is small for his age so I guess I am ok.
Most go up to 13 or 15 kg and the next stage start at 9 kg, or now recommend 10 1/2kg for those using just the adult belt. As for height, if it is the same recommendation as for the forward facing ones,as long as his ears are below the height of the seat back he should be ok. dd was almost 18 months before she moved out of hers.
Yep, they're ok until their eyes are level with the seat back in a rearward facing seat.
The problem with moving them up to a bigger seat before they reach the lower weight for the seat, is that the seat won't have been crash-tested for that weight.
But dd was 18mo before she weighed 9kg. Her eyes were still below the level of the back of her infant seat. And the weight limit for her infant seat was 13kg - she doesn't weigh that now, at 3yo. But we moved her up at about 1yo when we could no longer stuff her legs in!