Everyone will be technical and tell you 6 months. But around 5 months I started both of my boys...they were both around 17 pounds and sitting well with assistance. Did the cereal and thickened it slowly then introduced veggies first, then fruits second. It's so fun.
6 months. I started with rice cereal then oatmeal. Also I did veggies first because I believe once they have fruit first they have a hard time with the taste of veggies. JMO. My daughter loves oatmeal cereal till this day. Plain with water.