I'm not sure anyone has traced where the 4-ish year figure for average weaning age comes from.  The best information *I* know of comes from Katherine Dettwyler, an anthropologist and friend of this list who wisely looked outside humans for answers.  She looked at various biological markers in other higher primates, compared them to weaning in those presumably culture-free species, then looked at when those same markers occur in humans.  Markers included comparisons with birth weight, adult weight, adult body size, gestation length, and dental eruption.  

Katherine came up with a "weaning window", during which human weaning could be expected to occur naturally and without cultural pressure, of between 2 1/2 and 7 years.  The WHO recommendation of a minimum of 2 years, long as that might seem to mothers in the US, doesn't quite make it to the bottom edge of that window.  But it's a good start.  Fully adult immune competence, she says, begins around age 6.

Here's the link to her article for LLLI:

www.llli.org/ba/Aug94.html

Diane Wiessinger, MS, IBCLC  Ithaca, NY  USA
www.wiessinger.baka.com

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