chiccadee, I thought I'd read it on MN, but just looked it up online and it's not as clear as I thought.
I'm in no way an expert on this, I've just had a quick google.
International average is sometimes quoted as 4.2 years old, but this may not be accurate. What I found was the suggestion that anything between 2.5 and 7 years old would 'fit' for humans as mammals.
A natural age for weaning
Natural term weaning then would look at what the biological imperative would be, rather than the actual world average (although there are plenty of cultures where it is normal to bf to 2 year, 4 years, even 7 years).
I also read the suggestion that historically breastfeeding has lasted for 2-4 years (for one thing it was a good way to space out pregnancies).
Anyway, I don't claim to know any of this as fact, and stand a little corrected on my earlier post.