I like to have the nests open and available before they start to lay so if they start pooping in there, sleeping in there, or scratching the bedding and fake eggs out, I can fix the problem before they start to lay, not try to fix it after. And I want them to have every opportunity to lay their first egg in a nest.
Its not all that unusual for a pullet to drop her first few eggs from the roosts or just walking around, but when they develop control they will look for a safe place to lay. They are critters of habit. Once they lay that first controlled egg, they often want to lay all their eggs there. They seem to like protected corners or dark places, but just shadows on the coop floor can make them think the place is well hidden. Its kind of surprising how many actually get it all right from the very first.
Ive had pullets start to lay at 16 weeks. Not many, but a few. With the chickens you have Id certainly be ready by then.
Put a fake egg in each nest. They like to lay where others are laying. I think it makes them think its a safe place to lay. I use a golf ball but other things work, like wooden or ceramic eggs. I even used plastic Easter eggs once, filling them with sand and gluing them shut, but they broke apart a few months later. They go through some punishment from the hens scratching to rearrange the bedding material and get the nest just right.
I dont have any choice now. I have older hens laying so I have to keep my nests open all the time. But the nests were open for my first batch when they left the brooder at 5 weeks. And when they started investigating the nests before they started to lay (18 weeks for my first one that time) I found I had a problem. They were scratching the bedding and fake eggs out so I had to raise the lip to hold that stuff in.