They usually start around twenty weeks if the light is sufficient when they reach that age. That's not specific to Khaki Campbells but to domestic ducks in general.
I have to confess I was telling people out the wrong age (16 weeks) until I looked it up again.
Personal experience light didn't have anything to do with laying. At least not with the ducks. They laid perfect eggs from the beginning and then started laying regularly pretty much daily unless broody or molting. Ours were about 5 1/2 months old when they started laying and that was in January. The only thing I've been warned about was a sudden change in environment. This can make them go through a molt and stop laying. I've read that someone added a light when their girls started laying causing a molt then no more eggs.