Milk is a maternal lactating secretion, a short term nutrient for new-borns. Nothing more, nothing less. Invariably, the mother of any mammal will provide her milk for a short period of time immediately after birth. A familiar example is that of a puppy. The mother nurses the pup for just a few weeks and then rejects the young animal and teaches it to eat solid food.
Nursing is provided by nature only for the very youngest of mammals.
Of course, it is not possible for animals living in a natural state to continue with the drinking of milk after weaning.