Merging ducklings at 2 different ages

[WendyK]

I'm new to raising ducks. I have 2 sets of ducklings due to loosing 4 out of my first set. The 2 that survived are now 5 weeks old. I ordered 5 more, thus 2nd set. They are 2 weeks old. At what point can I add put them all in together.

 

[Bills vs Beaks]

When they get to be the same size, so everyone is equal and they wont pick on each other