4 to 6 months. Before starting any solid food you should make sure your child can sit up straight (it's ok if it's with the support of a high chair) and that they no longer have a tongue thrust. To test, move a spoon towards their open mouth, if they thrust out their tongue they are not quite ready for solids. Try again soon! Feel free to start a whole grain cereal or even a veggie as a first food.
4 months. Towards the end of the 3rd month when the infant demonstartes good head control and if weight gain has been acceptable without issues with feeding, the transition is made to rice cereal mixed with either the formula or breast milk expressed and offered with the second feeding of the day.
4-6 months of age. Most babies should be introduced to rice cereal between 4-6 months of age. The trend today is at about 5-6 months old.