My baby is also five months old, so I went to do some web research into this question. I also purchased Super Baby Food, recommended highly by people here.
The World Health Organization did a study of what they call complementary feeding, or feeding solids while baby is eating breastmilk. Their researchers concluded that six months is the optimal time to introduce solid food. They decided this based on the idea that introducing solids too early could displace breast milk, which they (and every other public health authority I can find) think should be baby's main food. This is the WHO page on complementary feeding.
It contains links to several documents on their research and policies. Keep in mind that they work in some countries where people have a hard time getting enough to eat--though you can also see that we here in the US are a subject of concern for them when it comes to our baby feeding practices.
Nearly every public health organization and medical association in the US recommends exclusive breastfeeding until 6 months. This includes the American Academy of Pediatrics, the American Medical Association, the Centers for Disease Control--and some others I'm not remembering just now. If your doctor gives you a hard time, you can print out this 1997 position paper of the American Academy of Pediatrics.
Be sure to use a highlighter on #6 in the "Recommended Breastfeeding Practices."
Other experts say that the main indicator for when to start solids is developmental readiness. The current thinking is 4 to 6 months, but there are signs that the baby is ready, including the ability to sit up and the ability (and desire) to pick up food and put it in his mouth. Here's a nice clearly written page from the La Leche League FAQ on developmental readiness to start solids.
My SIL's children were slightly delayed in some of their motor skills--they walked late, for instance--so she delayed solids until 7 months, and her children did not lose weight or fail to thrive.
The main source of your baby's calories should still be breastmilk through the first year, as I learned from my web search and other reading. It seems like it is important to introduce nutritious food, and that delaying solids indefinitely isn't a great idea, but there isn't a rush.