when to start oatmeal

My little one is only 3 month so I know not right now. But when are you supposed to start giving it? My friend gave us a box full of baby foods and gerber oatmeal. I remember with my first that we started baby good at 4 months, her Dr said to start it once a day. But that was 11 years ago. She has an appointment with her Dr in a few weeks and I plan to talk to her about it. My daughter holds her head up, can push up on her arms when on her tummy (though she just rolls over to her back usually) and is a supported sitter. Not sure if any of that matters.