I can't be sure of the exact age, but I think a duckling's oil gland comes in at 2 weeks of age. Once the gland comes in you can put them in warm water for awhile every day, just make sure they dry off quickly and stay warm. Once they start losing their fuzz and begin growing in real feathers, they can stay in the water longer.
We let or ducklings have 5-10 minute supervised bath time (warmish water) at 3-4 days old. Its very important to not let them stay in to long or the can get water logged. after bath time make sure they have a warm place to dry out.
