Each one of mine was different... my 6 yr old was about 7 mo, my 4 1/2 yr old was about 6 mo, and my 2 1/2 year old was only 3 1/2 or 4 mo old. They all were a little different in gaining weight & showing signs of being ready to eat.
Things to watch for are... 1) are they able to hold up their head without support 2) have they doubled their birth weight 3) are they showing interest in what you are doing when you eat - opening mouth when you do, trying to grab food when you are eating, moving mouth like they are chewing when you are chewing 4) are they 6 mo of age... as long as they show multiple signs they are ready to start eating cereal & baby food.
When you do see the signs & choose to start giving them food & cereal - understand that eating from a spoon is a new skill that will take time to master. It might not be easy to begin with, but with practice they will get better. Try not to feed them when they are super hungry or they will not even try. Also, only try one new thing every 3-5 days... this spacing of new things is to watch for allergic reactions to foods and do the single item foods before you try them on the multiple ingrediant foods. Oh, and when you first start make the cereal super runny (I mixed w/ formula or breastmilk) that way it is easier for the baby to eat it - since the texture is a bit different & will seem weird to them.