C. Martin (you neglected to add your first name)
        Your questions are very good ones and as you see there are no absolutely
definitive answers in the literature.  As I see it, breastfeeding is just
about always beneficial and ideally should continue until the
nursing-dyad feels that it is OK to stop.  This is generally a few years
down the road.  You might want to look at Kathy Dettwyler's work on
duration of breastfeeding.
        In the infant and young toddler the situation
is a little bit more complicated.  Many reccomendations are not research
based and reflect the biases of the person or organization, making the
statement.  Breastmilk always has physiological benefits for the
nursling.  Although it is true that the relative importance of these
benefits change as the child gets older they never disappear.  Exclusive,
total breastfeeding is sufficient for virtually all infants for
approximately six months.  Many infants can thrive on breastmilk alone
until almost 1 year and possibly even longer.  Breastmilk provides
significant nutrition for a nursing child for a number of years longer.
Many mothers have found an enormous increase in the quantity of solid
food eaten by nurslings after weaning at ages 3, 4 and beyond.  This
underscores the value of the breastmilk even at a later age.

Hope this helps.

Sarah Friend Barnett   LLLL, IBCLC
Bronx (New York City), NY  -  [log in to unmask]
" You are not obliged to finish the task,
 neither are you free to neglect it."       R. Tarfon