I think the general rule is not before 4 months at the earliest, but the NHS recommendation is to wait til 6 months to wean, though certainly people start before. We did BLW and never did any baby rice or pureed foods. Started with normal food, like roasted vegetables, roast chicken, toast with hummus or banana or avocado, rice cakes with cashew butter, etc. at 6 months. When we did do porridge, we found just using normal adult porridge worked just fine, but you should wait until 6 months to use an oat-based porridge as babies shouldn't have grains like wheat, oats, barley, etc. before then.