It really depends on what you are going to do with them? Ducks will start to lay around 20 weeks and would benefit from going onto layers pellets from around 16 weeks. You can feed poulty corn now and start them on grit or oystershell too. Quite often we have swopped from crumbs to hand crushed lauyers pellets and corn, much cheaper and easier. But if you are raising them for feed then you might look at finishing pellets soon.