How old do ducks have to be to start breeding?

We got our ducks as day olds the end of May. After their molt in August we knew we had a drake. He has a special liking to a particular chicken and now it appears he is trying to do the deed with her. I know I've read that a duck can kill a chicken in the process and we have plans to separate them this weekend, but I'm curious that he's already trying to breed. I didn't think we'd have to worry about this until next spring. Is this normal in a bird that is so young?