Yes, women should breastfeed their children after they are considered babies, if they want to. Breastfeeding is healthy. It is the most healthy food that children can have. Society has decided on an arbitrary age at which this needs to stop. But breastfeeding is natural and should be encouraged into later years.
By age 4, 5, and 6, women should not be breastfeeding the child for a few reasons. One, it breaks a societal taboo to have it occur. Two, it may cause psychological trauma to the child to be seeing their mother naked at that late of an age. Lastly, at that point in their lives the child should be deriving their nutrition from actual food.