Either or works. For my first chickens, I didn't add the nest box until they were pretty close to laying. But if you want to add them now, there's really no detriment - if the chicks want to sleep in the boxes, just block them off for time being. If they leave them alone, then no reason to block them.
My nest box was in place before they arrived and as soon as they had access to the floor they had access to the nest box. The most important thing is that the perch is higher than the nest box so they don't try and sleep and poop all night in the nest box.
You can see the nest box below the poop board in this construction photo.
Photo from a few days ago, almost have all the brooder removed in this photo.
In this photo you can see one of them exploring the nest box area. Sometimes there is several in there during the day.
I had my boxes built and screened off.
I was going to open them when they got ready. I was a day late. As soon as I had pull the wire off she flew in the box. So 16-18 weeks old? go for it. Less than 16 weeks old you've not getting eggs anytime soon.