6 months is the recommended time frame. Rice cereal in formula or breast milk can be mixed in as early as 3 months.
Between 4-6 months. You want your baby to be mostly able to sit up on their own before giving any food. Other signs that your baby is ready to start eating is being interested in what you and your family are eating, and mimicking you as you chew. You then want to make sure it's puree'd and use only cooked food- no raw.