My research (and I'm the ONLY person, so far, who has done research on this
topic) indicates that the "natural" age of weaning for humans ranges between
2.5 years as the absolute minimum, and 7 years as the maximum, with many of
the indicators falling at the 6-7 year end of the range.  There is no
scientific evidence to suggest a natural weaning time of 6 months or 9
months or 16 months or 18 months or even 2 years or 2 years 3 months.  I
think that many mothers give LOTS of signals to their children, whether
consciously or unconsciously, that they don't really want to breastfeed any
more -- giving them more solids than are really appropriate, offering water
and juice, suggesting that the child wait until the mother is less busy, in
a less public place, in the mood, etc.  I think it is the rare baby who is
truly ready to wean, physiologically, before 2 or 2.5 years.  Obviously,
with plenty of adequate other sources of nutrition, few diseases, and very
little in-arms time, many children are able to survive and thrive without
extended breastfeeding.  That doesn't change the underlying physiology, or
indicate that there is anything "natural" about weaning at 16 months.

Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University
Author of "A Time to Wean" in the book Breastfeeding: Biocultural
Perspectives