This article has some interesting findings on the benefits of nursing babies and how it impacts their brains from the very start. It also touches on the shown benefits of nursing a child past one year in age.
Many times I've had parents ask how long they "should" nurse and the correct answer is there is no "should" in nursing! It's truly whatever is best for mom and baby, recognizing that the nursing relationship needs to be supported by the entire family unit as well. But evidence is now showing that ANY amount of breastmilk is beneficial to a baby even if it's the colostrum of the first couple days or a couple years of milk. Research is also showing evidence that nursing benefits last a life time, and nursing past a year IS beneficial for children.
So remember, nursing should continue as long as mutually beneficial and breastmilk doesn't ever stop benefitting your baby.
(Photo by the lovely Mae Burke of me nursing my 21 month old explorer.)