Watching with interest - DS is 13Kg at 7.5mo. We've just had to shell out £200 for a car seat which will fit him (the industry thinks 13kg is a year old) so I expect we'll come up against this one too.
it depends on the child and how much you use a pushchair (i.e. if you use the car a lot instead). We use a pushchair sporadically for ds who is nearly 3 but dd stopped using one before she was 2. I do have a car though so I dont often have to walk long distances.