Be careful about integrating ducks of different ages--if there is a large size disparity, the smaller ones may get picked on severely, no matter how careful you are about introducing them. However, once they are roughly the same size you can integrate them like any other duck. I sometimes just throw them together and see what happens. If there is a lot of fighting (beyond just pecking order establishment), I separate them and place them in pens where they can see each other but not reach each other. After a week or so of that, I try again, and usually it's fine.
Good luck!