My pedi say 4 months to start, however I gave my son some carrots and bananas when he was 3 months. He did just fine with it. I only gave him a few bites. But I've also heard oatmeal is better than rice cereal.
Babies cannot digest foods other than milk/formula before 6 months! Their digestive system is simply not ready for it.. Showing hunger signs when we are eating means they are learning social norms and making that connection..
I started giving my son rice cereal at 3 months. Because he was wanting to eat every hour. It helped a lot. And then started not needing it anymore after a little.