A nursing dog doesn't have sour milk. It produces continously, as long as she nurses. If for some reason she has full teats, but no longer feeding, the milk is absorbed back into her body. It's called drying up. Some females have extended teats. That happens when pups nurse, and there is no milk to be had. Age does not matter. A nursing mother is just that.
If you mean for feeding pups,it's perfectly fine for older ones, but not the newly born pups as there isn't enough nutrition in it.