DD1 we started around 10 weeks. She was taking over 40 oz. of formula a day and her pedi recommended it because obviously formula alone wasn't satisfying her. She's a happy and healthy 4.5 year old. DD2 is 8 weeks and barely taking 24 oz. per day so she is good with just formula for a while. I think she needs to be taking at least 4 oz every 4 hours MINIMUM before she takes anything else. She's barely making 3 hour feedings without needing a little snack in between.
Six months, but baby 'cereal' is fake food. When your baby is six months old you can go right to vegetables. Sweet potatoes, peas, squash, even broccoli, just steam and mash (microwaving zaps the nutrients). It does not have to be puréed, just soft. With my kids I introduced a new vegetable for about 3-5 days to make sure there were no negative reactions possibly signaling a sensitivity. I learned that cereals and other seemingly typical baby foods that we were brought up on are not necessary. Today we know that fresh food is best (preferably organic) so I encourage you to check out info on how easy and inexpensive it is to feed your baby healthily. Thank you and good luck!