I've always understood that the time for a baby to start solid foods is
sometime after 4 - 6 months (closer to six, or even later if the baby is
thriving on mother's milk alone) when the tongue protrusion (also called
extrusion) reflex has disappeared.  To test this offer the baby a quarter
teaspoon of any suitable food (mashed banana is good).  If the food goes
from the front of the mouth to the back of the throat and is swallowed then
the baby is ready for solids.  If, on the other hand, the food is spat back
out, and mother has to scrape it off the chin and stuff it back into the
baby's mouth (I think we've all seen this, haven't we?) then this baby is
too young for solids.  Wait a week or two and then try again, i e watch the
*baby*, not the calendar.

A good description of the tongue extrusion reflex and the rationale for
delaying solids pending functional maturity can be found in Chapter 4 of
Infant Feeding: The Physiological Basis.

Pamela, Zimbabwe