It is recommended to start at about 6 months old. You can start as early as 3 months but a lot of babies aren't ready at that time.
I started my son at exactly 4 months. He showed all the signs of being ready and I could tell he was literally hungry.
My daughter right now is almost 4 months but she isn't ready yet.
Watch for their cues....they will watch you eat, make mouth movements like they are eating.
You also have to take it slow...starting with the rice cereals and move on from there. Lots of info and charts that show when it's best to start certain foods.