I wait until sitting. Studies of various populations show that breast milk alone supplies balanced nutrition for the first year +. Most early solid feeding pressure comes from the days babies were fed home made blends of boiled cow milk/karo & vitamins.At the time early supplements made sense, now they are overdone.Starting should focus on training baby to sit up & eat in preparation for his/her position at the table.
At 4 to 6 months old. It's best to wait until your baby isn't filled up enough with breast milk or formula at each feeding. When it's time, start with a brown rice baby cereal (more nutrients and fiber than white flour), mix it with a little water, breast milk, or formula, and give it by spoon, not in the bottle. Gradually increase the number and volume of solids as your baby's doctor recommends.