If they are 17 to 20 weeks old, they could start laying any day. Of course, it could also be another month or more. You never know. I'd want to keep the nest boxes open since if they ever get in the habit of laying somewhere else it can be difficult to change their habits, such as you are experiencing now with their roosting.
Can they get to the roosts? Mine can and some do jump/flap up 4 feet to get to the roosts but most hop on top of the nest box which is about 2-1/2 feet high and then hop/flap over to the roost. They all jump/fly down in the morning and ignore the intermediate step. With your breeds and their age, they should easily be able to handle 4 feet or more, but giving them an intermediate step on the way up would not hurt if your roosts are fairly high. I seriously doubt that is your problem, but I'll mention it just in case.
Are your roosts noticeably higher than the nest boxes? If yours are close to the nest boxes 6 inches may be enough difference but if they are across the coop in a fair sized coop, it may take a foot of difference for them to realize which are higher.
Some people may come on and say that your roosts are not wide enough, to lay a 2x4 flat for them instead of standing it on its edge. Mine roost on tree branches instead of 2x4's. Parts of those tree branches are thinner than the thin side of a 2x4 (1-1/2") and very round. The favorite spot is by the window where the tree branches are thicker, but the second-favorite spot is where the branches are thinnest, less than 1-1/2 inches thick. I very much doubt the width of your roosts is the problem.
I think your best option is what the others have suggested. After they go to sleep at night and it is too dark for them to see, move them from the nest box to the roost with as little light and commotion as possible. You may have to do this a few nights, but they should get the message before too long. If you have lights in the coop where they can see to move after you place them on the roost, I'd suggest making it too dark for then to see to move.
Good luck!