I voted 4 months - it was acutally about 4.5 when we started but I had been considering it for a couple weeks before. Since ds had a lot of trouble with gas and has reflux I was planning to wait until 6 months, but he's so big the doc told us to start right then, he needed it. And boy did he - he's a champ when it comes to the solids, and has no trouble with anything.