Is it bad to breastfeed until you are three years old?

Yes: The developing child requires more nutritional supplementation by age 2 and 3. Moreover, there ae pyschological maternal "issues" when "the child can now ask for the breast" to continue feeding. Most passive immunity passes from the mother to the infant during the first 3 to 6 months of breast feeding. Usually, most mothers are done breast feeding by 12 to 18 months of their child's life.

Define bad ?: Even in the bush, where nutrition/ water sources are hard to come by, aboriginal cultures do not BF after age 2. The nutritional superiority of BF begins to disappear by age 8-9 mo. Retaining the process after natural weaning time does retain the nurturing aspect of closeness. If you want a kid to grow up self sufficient you wean them and challenge them to do things on their own.