I think most start around 16-24 weeks old. My Pekin started at like 16-18 weeks.
They will lay anywhere they want, however if you provide them with a nest box, I find they lay in them about 50% of the time. I find my girls lay early in the morning, so I let them out around 8 or 9 am and by then I can collect the eggs in the pen, so I don't have to worry about doing a treasure hunt through the lawn for them.
good thing I don't have a big yard they have access to lol.
Right now they are in the chicken run, and I'm considering separating that part of the yard so the ducks have their own area. Though one of the chickens has become very good friends with duckies, cuddles up to them, follows them, even crawls under them when she wants to hide, and yeah, the ducks let her.