The solids at 4-6 months general rule of the World Health Organization's
Infant Feeding Recommendations since 1979 is based not on iron stores, but
on growth rates of children around the world.  In Third World countries
(First World = Western Industrialized, Second World = Communist peasant,
Third World = everybody else) -- the average growth begins to falter at
about 4 months if only getting breast milk.  In First World countries, the
average growth begins to falter at about 6 months if only getting breast
milk.  This is AVERAGE.  Obviously, some babies will have growth faltering
earlier, and some later, on exclusive breast milk diets.  Some will be fine
up to two years on breast milk alone.  The WHO recommendations suggest that
each mother consult with her health care professional to decide what is
appropriate for her child, based on its health, growth, etc.

Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University