Before four months, you run the risk of getting allergies and foods can be a choking hazard. If you wait much longer than seven or eight months then babies get really used to full liquid diet and may have a harder time adjusting to thicker textures.
Doctors used to recommend a very rigid order for the introduction of foods, but that's no longer the case. Now they usually suggest that parents offer their baby cereal, fruits, vegetables, or meats in whichever order they choose.
Sometimes children have a bad reaction to what they eat, so wait a few days between the introduction of each new food.
When starting solids, it can be helpful to start the new food in the morning, that way a parent can watch a child for reactions during the day while the parent's still awake. You might expect reactions, such as a skin rash or hives, vomiting, or diarrhea, if a child has a food that does not agree with him.