I breast fed my 1st [cos u get pushed into it] i could only manage a month tops - gee it was painful & blimey couldnt handle the demand of my boobs - even blokes dont take that much time over them hehehehe
We had a mum at nursery who's 3/4 year old was still suckling & wasn't quite discreet either -

teeth on nips - yeeeeeeeeee-ooooowww bad enough with gummys
Basically as people said it is for the mothers needs not the childs, i've watched a few progs where mother is still breast feeding a child, & bascially the mother was one having problems letting go not the child - one mother gave up only for hours she sneaked off to breast feed her child totally confusing it at the same time, she had problems accepting her kid was growing up & doesn't need to be that dependant on her!!
Personally i think certainly before child goes to school then breast feeding stops or they are causing abuse or force because it can be quite psychological if that child is usually one or two that attends the school still suckling tantermounting to bullying later on. Usually most mums wean of breastfeeding by the age of 2 or even before then. If they have a good diet then no need for 'baby' milk of any kind.