The youngest I've had a pullet start to lay is 16 weeks, that's happened a few times. It's possible they could start laying earlier, with chickens I try to never say never, but I would not read too much into that. They may be dust bathing or they may just be relaxing.
Before a pullet starts to lay some of them will look for a good place to lay. Not all of them do that, for some the egg laying seems to come as a surprise. Sometimes they can start looking for a nest a week or so ahead of time. You also never know for sure when they will start. There are all kinds of clues they might be getting close but those can also be false clues. They may mean nothing. Or a pullet may start to lay without exhibiting any of those clues. There are a lot of myths and mythology about when they start but the only way to know for sure is when you see an egg.
Personally I'd have those nests open now. I would not want to train them to start laying somewhere other than the nests by having the nests closed when they start looking for a nest or start laying. Why create a problem by treating a problem that probably doesn't exist? I know you see that on here all the time but I've never really understood that logic.
They are already roosting. If the roosts are higher than the nests it is pretty unlikely they will switch. If the roosts are not higher than the nests they could switch. There are some things that could cause some to start sleeping in the nests even if the roosts are higher. If I were going to have a problem with then sleeping in the nests I'd want to know before they started laying so I could fix the problem before I got poopy eggs or taught them to lay somewhere else.
When they look for a good place to make a nest that usually involves scratching. If you find that they have scratched out your nest bedding and fake egg, that may mean you need to redo your nest. I've had that happen, I had to raise the lip a couple of inches to stop them from scratching everything out. I'd want to know that before I got a real egg that could break if it were scratched out so I could fix it timely.
If some are sleeping in the nests there is a reason. That could be several different reasons. If it does happen you kind of need to know why it is happening so you can come up with the right fix. If you do have issues when you open the nests come back on here and we can discuss that. But first find out if you have a problem. Worry is interest spent before it is due.