They should start laying any time now since the weather is warming up and days are getting longer. Are you sure of their age? If they are molting it may take them a little longer. It is possible for them to hide eggs from you and some birds are better at this than others so be sure to look around for them especially if you notice one bird going off by itself. Good luck!
I only have one BR and she's 18 weeks and not laying yet but my 20 week old PRs are laying. Just got going this week.
Amount of light either artificial or sun light they get during the day helps get them laying. Also if all you have is shavings, try straw in the nests. In the past I have had some with no interest in laying in shavings immediately lay when given straw.
I think jlmann is correct but I am new to fairly new to this myself. My pullets just started laying yesterday and they turned 20 weeks old yesterday. However, I think they started laying at 20 weeks because I timed it right when I ordered them and they just happened to turn 20 weeks in the spring. Good luck and keep us posted