We introduced our ducks and they were younger, but our "adults" were younger as well. They are only about 4 weeks older, but much much bigger than our younger ones! A system we use is, we have an outdoor pen that we allow the little ones to play around in during the day (kind of like those dog kennel set-ups) and then at night we take them back in, when they get old enough to start swimming in the big pond, we let them do supervised swims and if the other ducks come along, we watch, if somebody starts getting picked on we take the younger ones back into the pen. I tend to over-react when somebody gets picked on though, the older ducks were my parents and the younger ones were mine (2 older to 2 younger) and I have to admit mine weren't willing to take the bullying, they try to bully back despite how much smaller they are.