You can introduce anytime! At first hold the duckling in your hand and let the older duck come and investigate. After a while, If the older duck shows no interest, put the duckling down and supervise him/her.

Start by putting it out with the other duck and watch closely to see how the react. if badly put a small fence around the small duck till they get more friendly.

I have a week old duckling that I just got. I keep it inside for right now. I have a 4 month duck outside in the yard. When should I take and put it outside with the other duck? Would the other duck be mean to it cause it likes to chase the cats and dog around the yard?