You can open the nest boxes at any age if they are sleeping on the roosts. If you open them before they are in the habit of sleeping on the roosts, they may get in the habit of sleeping in the nest boxes. You really don't want that.
I recommend 16 weeks as a target age. It is possible that a pullet can start laying at 16 weeks. I had one that did, but most don't lay until later. You want the nest boxes open when they start to lay so they don't get in the habit of laying somewhere else. And, if you do wind up with some sleeping in the nests instead of on the roosts, it is nice to have some time to work on the problem before they start to lay.
You are not enticing them to start laying by putting the fake eggs in the nest boxes. You are showing them where another chicken thinks it is safe to lay, so they will think it is a safe place to lay when they start. It does not always work, but I am a firm believer that it does help a lot.