Mainstream wisdom generally says by age 1, or generally when they're getting the main of their nourishment from food as opposed to breastmilk or formula.

Breast feeding advocates recommend letting kids self wean, which generally means until the kids decide they're no longer interested - which can be anywhere from 2 to 4+ years old. Once the kids pass 1 1/2 years or so, the nursing relationship is generally more about comfort and closeness than it is nutrition.