breastfeeding untill….?

Now that many women are returning to breastfeeding there have been discussions on the appropriate age to stop breastfeeding ones child. I personally know moms who breastfed, at least part of the time, untill thier child was three or four.
Is there a certain age that kids should no longer be breastfed? Should it be up to mom and child regardless of age? If you object to a child being breastfed after a certin age have you ever said anything to a mom who was doing it after that age?