The idea of delaying solids well past 6 months and even past
1 year has been around for a while.  I am sure you remember that
waiting until 3 months was, not too long ago, considered very
radical.  I was sent home from the hospital with instructions to
start cereal at two weeks!  Only "LLL fanatics" waited longer
than that at the time. (He is now 23 and does have some
allergies).  It has been my experience that the research is way
behind the LLL subculture.
     Despite any emotional preferences I also have always wanted
hard, researched facts before sharing many things with mothers.
It has always surprised me how long I have had to wait for
confirmation of many widely held beliefs.
     I think that delayed solids has its roots in a few things:
1- If a little is good (breastmilk) then more must be better.
2- Desire to prolong lactational amenhorrea and concomitant
infertility for people who will not use other methods of
contraception.
3 - Desire to avoid allergies - Riordan (A Practical Guide...)
states "Delaying solids for more than 6 months in infants who are
highly allergic to a variety of foods is safe, providing the
hemoglobin level is monitored at intervals."
4- Waiting for developmental readiness to handle foods.  Riordan
also says that some cultures wait much longer to introduce foods
and sites that the Yuma Indians wait (waited?) until the infant
was ready to walk.  In the case of my son mentioned above that
would have been 17 months! (Yes he had solids before then - he is
also intact - he starts graduate school in electrical engineering
at the Technion in Israel in the fall - Maybe he would have been
smarter if I had nursed him exclusively/predominately for 17
months - oh well)
5 - Anecdotal reports:  I am sure that we have all seen and read
news items about the children who cannot tolerate anything other
than breastmilk for two to three years and who are depleting
supplies at milk banks.  There are also reports from individuals.
At the LLL convention at a session about nutrition there were
several anecdotal reports from parents about children who refused
solids, some until 22 months, and were fine.  At the same session
there was also a report about rural Guyana where solids are
generally introduced after 1 year with no apparent problems.
People tend to generalize from the one to the many, often this is
OK- we just need to remember all of the stuff about averages and
statistics etc. when evaluating this kind of info.
     I think that the jury is not in on the maximum length of
time that it is safe to exclusively/predominately breastfeed.  I
would like to have more cross cultural data.  That is an area
that I am not as familiar with.
     Hope that this helps and does not muddy the waters too much.
     Sarah Barnett
     Bronx, New York