I started at about 6.5 months with my 2. It reallly isn't necessary to start until 6 months as breastmilk (and formula) have everything the baby needs, at around 6 months BF babies need an iron source as they are starting to use up all the iron they stored from pregnancy, so it is best to start with iron rich foods such as pureed meat or iron fortified cereals.
In the beginning it is really about learning how to eat and transfer food rather than the actual food. If you do start early remember that the milk is the more important of the 2, much higher in fat and more important than solids so ensure that baby is not decreasing milk intake in favor of cereals.