With DS I stuck to the 'rules' at the time which was 6 months. My mum and grandmother couldn't comprehend it. Looking back now it would have done absolutely no harm at all to start at 4 months. By the time you get to six months, all of a sudden you are supposed to be trying to introduce all these foods when baby hasn't eaten at all. Intuitively I feel 4 months is good for cereals and very basic foods etc building up slowly over time and I'm going to do that with the next one.